Points to note.
1) Pro Tools does not use/run AU (AudioUnit) plugins, so the fact the EWI Software (whicb is actually Garritan's Aria) works with PT is a bit of a red herring...
2) Did you update the EWI/Aria AU Software to 22.214.171.124 which is the version that runs on Snow Leopard and later?
• Adds support for Mac OSX 10.6 Snow Leopard
It is available on the Akai EWI Docs & Downloads tab found on this page