On the character, click edit weapon (small icon to the right on the weapon stat row), there you will find a tab where you can select mag. This is how you do now. I don't remember if there will be a change when it is updated.
Thanks much. I thought I tried that earlier and it came up blank, but this time it listed the magazines the character was carrying. maybe I hadn't had them properly being carried. But now it works.
Does the program account ammunition usage?
Also, is there any Twilight 2000 specific Foundry VTT documentation anywhere? I'm so far learning by trial and error, and it can get pretty frustrating at times.