I asked Dartfish this same question.
The answer I got was the license key is only good for one download on one computer and it is not transferable to a different machine.
I’d suggest saving the license key for the machine you intend to use it on.
Also, be sure to submit your questions about the Dartfish evaluation to the FIRST Dartfish Software Evaluation forum as well:
http://forums.usfirst.org/forumdisplay.php?f=1322
The Dartfish staff is monitoring the forum in an effort to assist the evaluation teams.
Happy Dartfishing!