For those who are having issues with the MLRS SOM example mission or addMagazine not working. Try this: (Steam users)
1. Delete these files in your addon folder:
editor.pbo
editor.pbo.bi.bisign
modules.pbo
modules.pbo.bi.bisign
2. If you have to run verify integrity of game cache do so now.
3. Restart steam. Steam reload the files.
4. Run A2 and try ARTY again.
This worked for me, but I have not confirmed it for others who have the same issue.