Hi Guys!
Today I installed Cocos2d-x and begin to to some test.
Immediately I noticed that the damping on PhysicsBody does not work.
I figure out the lines 776:781 in file CCPhysicsBody.cpp were left commented!
Un-commenting these lines damping seems to work fine!
I sent a bug report on GitHub but decided to report also this information on the forum to avoid troubles to cocos users! 
Bye