If you are absolutely sure that you have not installed any Service Packs for your AutoCAD version and you already are on Yosemite, than you need to download new full installer fro AutoCAD 2013 from here:

http://knowledge.autodesk.com/support/autocad-for-mac/downloads/caas/downloads/content/autocad-2013-...

This installer already includes all Service Packs:

This full version re-release of AutoCAD 2013 for Mac is provided for customers who require a fresh install on Mac OS X Yosemite, since the original installer cannot install the product on Yosemite. This full version is in line with the AutoCAD 2013 for Mac with Service Pack 3.

Than remove AutoCAD 2013 installation you have on your Mac with "Remove AutoCAD" application,

than install it again with installer you downloaded now.

During uninstallation you can leave your AutoCAD preferences and after you install it again everything should be "in place".

Maxim