It seems that you are having some issues with the Slingbox since it does not allow you to select the right channels when using the SlingPlayer for iPhone application.
The fact that the Slingbox works fine on your computer lead us to think that the issue might be related to the SlingPlayer for iPhone application, otherwise, you would get the same behavior on your computer.
With this in mind, I encourage you to uninstall and reinstall the SlingPlayer for iPhone application and see how it behaves. If you do not know how to uninstall and reinstall this application, I encourage you to check this link:
Hope this helps!