I see that you are having difficulties getting the regular channels on the Slingbox, however the DVR options work.
Since your Slingbox is working on your iPhone and the computer, the issue can be caused by the application itself. Please, let us know which iPad version are you using.
You can also try to setup the Audio/Video Configuration, from the Setup Assistant on the computer itself. As well, verify if the Slingbox that you have works with the iPad, and if the issue persists, try resetting the Slingbox and configure it again.
Feel free to access the following links, those will help you with the troubleshooting:
I hope that helps!