When you selected DP01 - Stronghold as the map to base your mission on, did you keep all the zones that came with the map by default? If so, you'll have all the zones required for multiplayer use (all the spawn points, the central area etc).
If you have these, then it probably means you've missed a check box somewhere - so at least it's easy to put right when you find it! You should have boxes checked for single player game type, co-op, solo and team game types.
I think these are the ones in the edit/mission box you mentioned. There's also one that's something like "available to custom gametypes", and that should be checked too. I don't think that's in the edit/mission box though, it's one of the other two boxes in the edit menu. One's called briefing (it's not in that one I don't think) and there's another one there as well called something else... Map? It might be edit/map. Try in there.
Sorry it's a bit vague, but I don't have Igor on my computer at work, only at home. And I can't quite remember what it's called. I'm sure you'll find it though...
I've had a similar problem myself - unfortunately after I'd sent the mission to a few people who I knew wanted to try it out in MP! Ooops...
