I have had various issues with associating files with applications ever since windows 7, and I think the most reliable way I’ve found, besides using a third-party utility is:
- right-click on a .bsa file and hit Properties
- click the Change button, for the Opens with property
- browse to the bella_gui.exe file and choose it
To find bella_gui.exe, since it is unzipped by sketchup somewhere in your sketchup plugins directory, right-click on it in the task bar (I am assuming you have pinned it to the taskbar), right-click on its name in the menu that pops up, and choose Properties — you will find the path to it in the Target locatio property, or you can just click the Open file location button.