It could be that the key file is not in the Terragen 2 application folder, although it would normally be able to find your backup copy if you already had a previous build of TG2 running OK.
TG2 *should* show a message box when it can't find the key file, but I think this may use a part of the message box system which was accidentally disabled in this version.
Do you still have an older TG2 build to test?
Matt