One great way to encourage your community to add to your game is to make it easy for players to get those additions. It is not enough to have tools for modders; you want it to be easy for players to use those mods, which will encourage their use, which will encourage their development, virtuous cycle ho!
Having it in-game helps. If I can access player-made mods, maps, quests, characters, etc. without searching the web and planning ahead, I am more likely to do so. The trivial inconvenience of downloading something and putting it in the right folder will keep many of your players from using much of anything beyond the standard game. The hardcore MMO player who will crawl through barbed wire for his fun is an atypical user.
One great gift to modders in Torchlight is having an achievement for using mods. People will do absurd things for useless badges and shinies that no one else will ever notice their having. A player who might never have looked at mods will notice when there is an un-checked achievement box to try them. It is a tiny thing for the developers to do, but it adds much encouragement. Yes, it also rewards “do it once to get it out of the way,” but some people will find interesting things in that try.
City of Heroes combines the two with Architect Entertainment. There are in-game sites devoted to trying player-made content. There are badges awarded for doing various things. After the initial release, the developers even toned down the badges to “try it” rather than “grind it until you hate the system and play only farms.”
Ease of use plus a low-cost nudge: encourage modders and harvest their ideas for official releases.