The file type for BC Framer®, Structure and Sapphire Viewer are the same (*.mmdl) so the default program that opens when the file is double-clicked is set to the most recent install. This can be changed manually by editing the registry. (Editing the registry may potentially be harmful.)
Launch the registry editor. Click the Windows Start button and type "regedit" into the search box. Click on regedit.exe or press Enter on the keyboard.
Here is the registry key Windows uses when choosing the program to open MMDL files:
Double-click on the default key, and edit the path to BC Framer* (or the program you want to use). Be sure not to remove the space and command line switches at the end of the data string. (If the command line "/j%1" is omitted, a double-click will open the application but not the file.)
*Your program path may be different!
This is the default Framer install path*:
C:\Program Files\Boise Cascade BC Framer\Programs\UI.exe "/j%1"