If the tree view is important to you, just to know that version 1.9.x no longer has that. (Why would anyone take that out?) You will need to downgrade back to the 1.8.1 version. Luckily downgrading is painless, just drag-n-drop.

To download 1.8.1
http://downloads.atlassian.com/software/sourcetree/SourceTree_1.8.1.dmg

Ref: https://answers.atlassian.com/questions/289308/how-do-i-get-the-file-tree-back