Incase anyone ever runs across this and needs steps to help resolve a file association issue in the future:

If you click on a .sgu or .mgu file from the File Explorer and it doesn't load with Band-in-a-Box, please try the following:

1. Go to the File Explorer, and locate an SGU or MGU file that you've created.
2. Right click on it, and go to Properties, then click on Open With
3. Then go to More Apps (or “Pick an App” or “Choose another app”) | Look for Another App On This PC. These would show when you scroll down to the bottom of these windows. If you're prompted with the option to specify ‘Always’ or ‘Just Once’, choose 'Always'.
4. From here, you will want to locate bbw64.exe (or bbw.exe if you would rather run the 32-bit version). It would be inside of your bb folder.
5. Click Apply.

You will want to do these steps for both an SGU and MGU file format so they both can open the program when you select the file from the File Explorer instead of using File | Open. If the song opens when you click on the SGU/MGU file, the issue is resolved.

Cheers,
Ember