It seems you can install TextMate bundles in several different places and it will find them:
/wherever-you-installed-textmate/TextMate.app/Contents/SharedSupport/Bundles
/Library/Application\ Support/TextMate/Bundles
and I expect the equivalent in ~/Library works too.
To install:
mkdir -p /Library/Application\ Support/TextMate/Bundles
cd !$
svn co "http://macromates.com/svn/Bundles/trunk/Bundles/Ruby Haml.tmbundle"
Then reload TextMate Bundles with Bundles -> Bundle Editor -> Reload Bundles.
Note that GetBundle as described here seems unavailable/broken at present.
References:
The TextMate Manual, Bundles Chapter
Installing [...]