If you want to exclude all directories by default and only include specific directories like me use this:
exclude /
include foo/bar
Another Hint: As mentioned by danchr in the ticket #175 your next pull will not obey this filemap. So make sure you place your filemap in .hg/svn/filemap and add this to the .hg/hgrc of your clone:
[hgsubversion]
filemap = .hg/svn/filemap