Is it Protools standard or ultimate? It seems only 1 of the 2013 imacs are supported for ultimate.
Supported Processors (Pro Tools | Ultimate, Pro Tools | HD & Standard):
Late 2013 27" iMac14,2 Quad-Core i7 'Haswell' 3.5 GHz
Supported Processors (Pro Tools Standard only):
Late 2013 21" iMac14,1 Quad-Core i7 'Haswell' 3.1 GHz
Late 2013 21" iMac14,1 Quad-Core i5 'Haswell' 2.7 GHz
http://avid.force.com/pkb/articles/c...m-Requirements