Hi, on my iMac the M-Audio Keystation 49e works great.
A test whit GB and al worked well (Just the USB connection)
Volume, Pichbender, Modulation, Octave up-down, all work fine.
Ore reset your 49e (see your manual for details) maybe there are some internal settings in the Keystation that mess up your connetction.
Try a testing using SimpleSynth (
http://notahat.com/simplesynth/
ore Midikeys (
http://www.manyetas.com/creed/midikeys).