You can read more in the News Forum, but I'll try to throw in the hows and where's in here...
1) There's a button/light next to where the profile button is (next to your logged in as name!)
Clicking on the light will take you to the IM home. There you'll see your messages, buddy list, and ignore list. Anyone on your ignore list will not be able to send you messages. Those on your buddy list will show which site(s) they are currently surfing and give you a quick link (their name) to send them a msg.
2) If you click on ANYONES profile, you'll see icons to IM them as well as adding them to either list. (Ignore and buddy.)
3) In posts you will notice a new icon to send someone an IM Message.
This should sum it all up...
When you have messages, the light by your name (next to profile button) will change colors and should be easy to spot.
This is all temp icons and buttons, but should suffice for the time being.
Enjoy!