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Hi,
If you have AutoCAD LT 2013 from Autodesk reseller or directly from Autodesk, than you need to install all Service Packs for AutoCAD LT from here:
http://knowledge.autodesk.com/support/autocad-for-mac#?cg=Downloads&p=AutoCAD%20for%20Mac&sort=score...
If you haven't installed Service Packs 1 and 2 before you upgraded to Yosemite, than you need to remove AutoCAD and install it again with installer you can see on the top of the list from the link above.

Maxim