You could connect the signal output of the Skeleton block to a "To workspace" Simulink block. This will save the output of your simulation to your current MATLAB workspace. You can then access joint data as a 20x3xN matrix (where N is the number of frames in your simulation and assuming you are using the Windows kinect runtime).
I'm trying the latest version with an Asus Xtion Pro Live, but the NID Skeleton block always causes MATLAB to crash. The depth and video blocks work fine however, and everything works fine with a Kinect.