If on a PC, iLok runs a service that is on a delayed startup, so if you try to run it right after booting, it won't run. In the Computer Manager under Services, make sure both the Bonjour service and the Pace Licensing Support service are running.
What a pain just to try something out though!
I'm lucky like Neil is -- not having had any problems with iLok since I started with it. (For a freebie copy of SoundToys' "Little Alter Boy" of all things...) I have 30 activations on my iLok key now!!! It has become pretty indispensable.