Where is the source tree in Sourcetree?
Upon click on "tree", gitk shows the directory tree for a given commit. Is there something similar in sourcetree?
Directory tree as of first revision (see lower right window frame)
Directory tree as of second revision
Browsing the source tree at any given revision is not available right now in SourceTree (v1.5.3).
A feature request has been submitted as bug #1021 in JIRA (the official Atlassian bug tracker).
It hasn't been implemented yet but it's expected to be released in 1.6.
You can register to JIRA and up vote or watch the issue if you want.
Choose File Status tab (at bottom) > All files (at top) > Tree view.