Here is what I recommend:
1) Make sure you have the IR cable connected to the Slingbox, and make sure it is positioned in front of the cable box.
2) Search for your Getbox's model number in the setup. If you can find it or something close to that, select it, and try the virtual remote. If it doesn't work, you can click "Back" and try a different one.
3) If you can't find a working remote, you can try uploading a custom remote from elsewhere. These custom remotes are called BIN files. Here is an article that should be able to help you with this:
4) If all else fails, you can post a remote control request here on our forums. I'll drop an article concerning this here:
Hope this helps!