Workaround (This is guesswork and only works 50% of the time):
1: Start Arma 3 (Modded but that shouldn't be the problem - see "Additional Information")
2: Enter Eden Editor
3: Place compositions that have been uploaded to the workshop (I had to subscribe to get my own in that tab)
4: Leave Eden Editor go to Multiplayer and join dedicated server
5: Hope to see two Custom tabs in the Groups -> Composition window.
6: Be able to place the compositions.
See screenshot below.
I do believe that the top compositions are "Zeus" compositions, but I'm not sure any more.
{F2529923}