Dialogs are the new PMs.
Click the Dialogs button near the top right of your screen to see your old PM (Private Message) threads if you had any on the old forums.
On the Dialogs screen, click the Start New Dialog button to invite someone to a new PM / Dialog conversation.
On the next screen, click "Add Participants" in the left column to search for other members to invite to your Dialog / PM private conversation. Only the members who start or are invited to the PM/dialog thread can view it.
Let me know if you have any troubles!
Julie