I had a 'lite' version of a daw that included a vst plugin that was usable only in that daw, so that is a possibility to consider. When registering dialog pops up with a directory, it is looking for where you have the plugin on your drive, so navigate to the directory for the plugin you are trying to register (Garritan in your post) and it should be a .dll you are looking for. Can't say what directory or subdirectory it will be in, but the dialog should default to looking for .dlls so that should be all you see if there are any. Double click it and if it is usable in BIAB that should get you on your way.
Good luck.