There's really no need for a keygen or other way of generating a key, if you have a decent firewall (such as Look'n'Stop 2.5). Just grab a key here:
http://pmblaster.com/registration/ and use one of them to register. When pmblaster tries to connect to the Internet to register/activate, click Block (or Deny, depending on which firewall you have). This will keep it from communicating your key info with pmblaster.com, but PMBlaster will activate anyway. Then just unblock pmblaster in the firewall and you're good to go.