Q:

How to register Source Tree on Mac? 

I want to import the license for SourceTree.  Atlassian saying that I should register first and then import the license. 

But I cannot find the registration function on the menu bar. 

A:

Atlassian changed the process in the latest release of SourceTree (2.2 for Mac).  Launch SourceTree for the first time and you'll see a prompt to log in.  Have or get an Atlassian account (it's free), log in and that's it.  Announcement:

Gone are the days of being prompted to register after 30 days,
  download a license file, license multiple copies for more than one
  machine or figure out a way to license SourceTree for your whole team. 
  Upon updating to the latest SourceTree, log in once and you’re done.