Is there some way to make Mailbox for Mac access my Mac "Contacts" for recipients?
According to the Mailbox website:

On the desktop, contacts are currently populated from emails that are synced with Mailbox. We don't have a way to import your contacts from your email accounts at this time.
We're planning multiple contact improvements down the road, including full contact integration in the future.

On a side note, you know about the ban on questions for software not publicly released, but you didn't search the relevant website for assistance before posting? Tsk, tsk. ;)