How can I get my posts to go directly to Twitter?
Go to Settings (top right corner) and click on My Twitter link. You'll be taken to another screen on Twitter to allow authorization.
How can I get posts on my board to go directly to Twitter?
The easiest way to automate posting to Twitter is by use of the RSS feed. Each board has an RSS feed that has been optimized to be Twitter friendly. There is a link to it right after the board name on the board page (the icon says RSS - pretty obvious). There are services available that will take the RSS feed and post it to Twitter for you automatically.
Here's a few that I'm aware of:
Of these, the one that I liked the most was dlvr.it.