I don't know that brand of router but you need to enable Port Forwarding (I believe Slingbox uses port 5001 so this needs to be opened) and then a TCP feed must be selected instead of Relay or SNATT. Are you using 'watch' on this site or a Slingplayer on your computer to view the Slingbox? If using a Slingplayer hit 'Shift', 'Alt' and 'I' which lists the type of feed.
You could try logmein.com which is a free program that will allow you to access a computer at the location of the Slingbox (you download it and so do they) and you could probably sort it yourself remotely. I guess looking up the router instructions online would help too.
That should make it work! Sounds complicated but it isn't that bad.