You're probably going to want to add the community to your friends list. After this, you'll want to log in and go to update, as you normally would. You'll notice under 'Optional Settings' (where you find the current music and current mood) it says 'Journal to post in'. If you click on the drop down, you'll see the community name in there, and your user name as the default. Click on the community name, and your post will show up here instead of your journal.
I feel like a 'Posting to a Shared Journal For Dummies' book.
(Recommendation for today is MOO Canada! ^_-)