two options mate, buy the game, or if you did, contact Lucasarts tech support for help, just don't go asking people for CD keys or cracks on here, forum rules prohibit that
I checked the GSA Forum searching for a similar problem (CD-key invalid at install), and I found that sometimes a firewall you're using blocks the authorization request from getting through.
Also, I'm just pointing out to everyone reading this that he didn't once in his brief post ask for a CD-Key. He just asked for help.