From the compatibility pages:
Mac OS X Lion Compatibility
Please Note: Pro Tools 9.0.4 and all earlier versions of Pro Tools software are not compatible and will not work with OS X Lion. With the following exception: Pro Tools SE 8.0.3 with Patch 003.
Looks like you might be in luck if you get that patch
(except it says its for Pro Tools
SE), but otherwise, you need to upgrade to 9.0.5 or PT10.