There are 3 things that could be causing an authentication failure:
1.
Entering the key wrong - Confusing 2 for Z or 5 for S is quite common. Check the original email rather than a hand written copy. Under windows, you can copy and paste the key directly from the email into the game. If you don't have the original email, you can have it re-sent to you using this form:
http://blockland.us/index.asp?p=keyRecovery 2.
Network access is Blocked - If you have any firewall or security software, make sure it is set to allow Blockland to connect to the internet. Anonimizer software that blocks PHP session ids or cookies can also cause this problem.
3.
Hard disk access is Blocked - On the mac version, be sure to copy the Blockland folder to your applications folder and run the game from there. It does not run properly from the mounted image because it is a read-only image. On windows Vista, you will need to run the game as administrator to gain the appropriate permissions (right click Blockland shortcut->properties->enable run as administrator->ok). Some security software can also prevent Blockland from writing to the hard disk.
