Hi Nicolas,
Definitely sounds like something is funky with your install. Lets do a clean wipe and start from scratch.
To clean out Max 6 entirely, remove these items:
1. ~/Library/Application Support/Cycling '74
2. /Applications/Max6
In Mac OS 10.7, the Application Support/Cycling '74 folder is hidden. You can find it using the 'Go' Menu in the Finder. From there, choose 'Go To Folder...' and enter:
~/Library/Application Support
then navigate to the Cycling '74 folder and delete it.
Then download and reinstall the latest Max 6 release: