Mike,
HyperLynx 7.5 was release well before Windows 7 was release and did not support that platform. I suspect it's unable to read the dongle ID in this environment. You could try updating the dongle driver to the latest by downloading and installing the latest licensing software (http://supportnet.mentor.com/portal?do=releases&prod=C106-S128-G159-P11292) but the license components HyperLynx was built with may still be too old to work on Windows 7.
The best recommendations I have are to run it on XP or upgrade to a newer version that supports Win 7.
Guy