Hmm, well it's just like posting on the forum:
First you have to make an account before you can post anything, or you can just lurk and read everyone else's comments.
There are smaller "boards" (subreddits) you can browse, each dedicated to a different topic, music, books, movies, artist, nearly anything you can think of.
Each subreddit has their own moderator(s).
Kimbra will be doing her Q&A, called "Ask Me Anything" (AMA), on the music subreddit.
Here's the direct link to the subreddit: http://www.reddit.com/r/music
You see all those threads? No different than the threads from here.
When the time comes, a new thread will be created for Kimbra's Q&A