Wifi password is too short on Mac Air in Lion
I assume you are being prompted for a WPA password. My SnowLeopard system (10.6.8) requires at least eight characters before enabling the OK button when I try to join a network using WPA.  The Wikipedia page for Pre-shared key (PSK) indicates that the valid password length is between 8 and 63 characters.

I also tried joining an example wireless network configured with WEP, an older wireless security method.  After entering just one character for the password, the OK button was enabled.

So, assuming you were given the correct password for the wireless network, it sounds like your computer believes that the wireless network uses WPA security when it actually uses WEP.  There's no way that I see to change the security protocol for the wireless network from the password dialog; your computer automatically detects the security protocol and sets the minimum password length accordingly.

Are you sure you are trying to join the correct network?  Does the password dialog specify WEP or WPA?  Do you know another person who is successfully joining the network, and can you have them disconnect and reconnect to the network, entering the same password?