In Terminal, navigate to the TextMate bundles folder.
cd ~/Library/"Application Support"/TextMate/Bundles/
If the directory doesn't exist, create it.
Clone the bundle - once you have it, reload your bundles in TextMate and it will appear in the bundles list.
git clone git://github.com/jackbrewer/css-plus.tmbundle
You can easily update the bundle through the TextMate menus by going to 'Bundles -> CSS+ -> Update Bundle via Git'.