You cant, unless you're an admin (currently that's just me and devtesla). My gues is this was intentional as a means to avoid things like subreddit takeovers and other drama on raddle. It could also be that emma (the dev for postmill) just hasn't developed a decent moderator hierarchy or something.

Since we're possibly a bit more laid back and small I think I'll figure out how to allow non-admin forum moderators to add other moderators. This will be a bit of a project so in the mean time ask me (or devtesla if he's up to it) to add moderators to your forum if you need them.