So today i received an email for access to the shootmania beta. I followed the instructions in the email and downloaded maniaplanet. After installation i opened maniaplanet up and was asked to create an account, which i did. After selecting my area etc i was asked to put the activation key in and i did (i received the beta key via email after i had downloaded the client). The problem is that maniaplanet returned an error saying the beta key is not valid.
I didn't have a problem with my key, however, I do have a question for you. Did you have ManiaPlanet on your computer before you installed Shootmania? I already had Trackmania 2 Canyon, so ManiaPlanet was already connected to my account. This might not be the reason why, but it might be helpful to see if others who have only Shootmania have this problem.
As for the key not working, you should try sending an email to the support here. If you have proof that you got the invite, they should be able to help you out.