Pop open your favorite shell and run...
cd ~/Library/Application\ Support/TextMate/Bundles/
git clone git://github.com/dsc/avro-tmbundle.git Avro.tmbundle
osascript -e 'tell app "TextMate" to reload bundles'
It's fine if TextMate is open.
Find a bug or want to contribute? Open a ticket (or fork the source!) on github. You're also welcome to send me email at dsc@less.ly.
This bundle was written by David Schoonover. It is open-source software and freely available under the MIT License.