...in a Subversion repository. You can browse it here:
http://svn.sourcesecure.co.uk/osflash/actionstep/trunk/
and the bug tracker is here:
https://secure.sourcesecure.co.uk/trac/osflash/actionstep
So to check out the code, use the Subversion client like this:
svn checkout http://svn.sourcesecure.co.uk/osflash/actionstep