Wow. I noticed that on Portforward.com had two completely different interfaces just for the Westell Versalink 327W. There's even another version on the Verizon website.
That really is the problem with all of these brands of gateways. There is really isn't any standard user interface. And a lot of companys will use their own terminology. So, its difficult to know what to do especially if you don't understand why you need to use port forwarding. So, if the above three links can't help then please entertain a long explaination.
Today, residential ISPs will usually assign one public IP address to each customer. However, most customers have more than one client that they want to access the network. In order facilitate this you have two basic technologies; Network Adress Translation (NAT) and a Private Address Space.
The Private Address Space is range of address which has been set aside by the Internet Assigned Numbers Authority (IANA). The address in space have been discribed in RFC 5735 (http://tools.ietf.org/html/rfc5735). The point of this private address space is to allow many organizations in different places to use the same addresses. In order to prevent IP addressing conflicts, these address will never appear in the public Internet. This means your private network cannot be addressed from the internet.
Network Address Translation (NAT) is a process that allows a single IP address to be translated into many addresses. In this case, it is translating a single Public IP address into multiple Private IP addresses. It does this by keeping track of which packets headed out to the internet and their destination. When it receives a response from the destination address it will understand how to translate the packet's destination address, so it can be accessed from within the private address space. This is great for when the communication originates from your network.
If the NAT service gets an unsoliced packet from the internet, it will not understand what to do with it or if it even belongs in your private address space. Port Forwarding is a firewall rule which can allow packets destined with a specific port of your public IP address to be forwarded into your private address space. It does this by scanning the port address of the packets, it will then translate the destination address to an address which you've configured.
Let's say your Slingbox has an address of 192.168.0.2. The typical port forwarding rule might look like "Port: 5001 to IP: 192.168.0.2". This means, if it receives information on port 5001 from your outside (public) IP address it will forward the information to 192.168.0.2 your internal (private) IP address. Some routers provide even more functionality by allowing you to configure IP, Subnet Mask, and Port for the External and Internal address.
You can create a firewall rule which will redirect all packets to your public IP with a destination port to a host in your private network. However, it will always direct it to that specific IP address. The problem is that most clients and routers will use Dynamic Host Configuration Protocal (DHCP). This is a service which will dynamically configure a network client with the information it requires to function within the network. This includes the IP address, so its possible that one day your Slingbox has an IP address for two weeks but something else gets its IP address. Remember if you have a port forwarding rule specifically going to an IP address it then it will always go to that IP address. Even if the address was obtained by a laptop.
In order to to prevent that, you can either set a static IP address configuration on your Slingbox, or setup a DHCP reservation. In either cases you'll have to need to assign an IP address outside of the range which your DHCP server is dynamically assigning. And in most cases you'll need to reduce the range of IP address that's available for dynamic assignment.
I know a lot of this sounds undaunting, especially to someone who doesn't work with network. But, I believe after reading this you should at least have enough understanding or buzzwords to scan through your router's different conficuration screens. You will need some information from your Slingbox for the configuration, like MAC Address for DHCP address reservation. Or IP address, Subnet Mask, and Port number for Port Forwarding. You can get this information by using the stand alone application from http://support.slingbox.com/go/windows-download-desktop-us From the Command Bar go to Help | About Slingbox <model> (Alt +E) | Information Tab and the information you'll need will be in the LAN subsection.
You also have another alternative. I noticed that your router is Universal Plug and Play (uPnP) compatible. This is a protocol which would allow your Slingbox to communicate with your router and set all the configuration for you. You would only need to find out where the setting exists and enable it within your router, then walk through the Slingbox Assistant for Internet Viewing in order to let your Slingbox take care of the configuration for you.
Hope this is enough to help you out, or at least get started.