We don’t currently package up binaries of any kind for BackPress, but you can access it via Subversion like this:
$ svn co http://svn.automattic.com/backpress/trunk backpress
If you’d like to get just the code (not the schemas for the database tables) then you may choose to use something like this instead (lines wrapped with \):
$ svn co http://svn.automattic.com/backpress/trunk/includes \ backpress
To use BackPress as an “external” in a Subversion-controlled project repository, you would use something like this:
$ svn propedit svn:externals .
and then enter something like this in the property definition file:
backpress -r123 http://svn.automattic.com/backpress/trunk/
where “-r123” would use a known revision number for stability. Exclude the “-rXXX” entirely to use the HEAD version, and stay up to date (but risk breaking!).