Amplitude should still run after the trial period. In the demo mode, it will still run however, it puts some static in the audio stream every few seconds rendering it useless for recording.
However... it came with MC6 and you, as the licensee of MC6 have the right to license Amplitude as well for free.
You do not register it on the cakewalk web site like you did with MC6.
You must open Amplitude and follow the instructions for registration. It involves going to the Amplitude server website and with a few clicks, you are legal and it will run unobstructed on your computer from that point forward.