You are unable to do a fresh install of the Mac version of AutoCAD or AutoCAD LT after upgrading your Apple computer to OS X 10.8 (Mountain Lion).
New Apple guidelines require that any software package installed on OSX 10.8 (Mountain Lion) meet certain security standards. In order to comply with these requirements, Autodesk has re-released versions of AutoCAD and AutoCAD LT that have been updated to include Service Pack 1 and conform with the new Apple security requirements.
If you need to reinstall AutoCAD 2012 for Mac you can download the updated version at the following link:
AUTODESK DOES NOT GUARANTEE THAT YOU WILL BE ABLE TO SUCCESSFULLY DOWNLOAD OR IMPLEMENT ANY SERVICE PACK OR WORKAROUND, OR ANY OF THE TIPS, TRICKS, EXAMPLES OR SUGGESTIONS OUTLINED IN ANY AUTODESK PRODUCT SUPPORT TECHNICAL DOCUMENTS. TECHNICAL DOCUMENTS, SERVICE PACKS AND WORKAROUNDS ARE SUBJECT TO CHANGE WITHOUT NOTICE TO YOU. AUTODESK PROVIDES TECHNICAL DOCUMENTS, SERVICE PACKS AND WORKAROUNDS "AS IS" WITHOUT WARRANTY OF ANY KIND, WHETHER EXPRESS OR IMPLIED, INCLUDING WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL AUTODESK OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF DATA, OR LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, THAT MAY OCCUR AS A RESULT OF IMPLEMENTING ANY SERVICE PACK OR WORKAROUND, OR ANY SUGGESTION OUTLINED IN ANY AUTODESK PRODUCT SUPPORT TECHNICAL DOCUMENT, EVEN IF AUTODESK OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.