First cd to your operating system's /Library folder (*not* ~/Library):
cd /Library/Application Support/TextMate/Bundles
Install the Haml bundle with:
sudo svn co http://macromates.com/svn/Bundles/trunk/Bundles/Ruby%20Haml.tmbundle
Install the Sass bundle with:
sudo git clone git://github.com/aussiegeek/ruby-sass-tmbundle.git "Ruby Sass.tmbundle"
Note: cloning the sass bundle without an additional directory parameter will not work. TextMate expects bundles to have the format "bundlename.tmbundle".