Autodesk does not support Mountain Lion [Mac OS 10.8] yet. For a list of Mac Compatible [native] products, you can go here:
Some of our appliations that are native to the Mac were designed pre-Mountain Lion [Mac OS 10.8] and you will notice that you cannot even install it. Please check out this article on one of the new features of Mountain Lion [Mac OS 10.8] called Gate Keeper:
Software installed on Mac OS X must be from an "identified developer" to prevent malware from being installed so please use this workaround with discretion.