Although most of our documentation says that Apple computers should be activated to the MAC address of your en0 network device, there are a few exceptions.
If you are using OS X 10.8 "Mountain Lion" or newer, and are using a Mac with more than one built-in network interface, you should use the MAC for your en1 network device instead. Some of the models that are impacted by this issue are: MacBook, non-retina MacBook Pro, iMac, Mac mini, and Mac Pro.
Models that typically aren't impacted by this issue are: MacBook Air, and Retina MacBook Pro.
Additionally, in some rare cases, MATLAB may switch between these two network devices on each reboot. If you aren't able to open MATLAB, but can resolve the issue after a reboot, this issue may be happening to you.
If this issue is reproducible, please contact MathWorks Support with both MAC addresses. They have the ability to generate a custom file that should help.
NOTE: In general, most activation issues can be resolved by running automatic activation instead of manually activating.