If you have configured your AirPort Extreme (802.11n) Base Station to use WEP (Transitional Security Network (TSN)), but join the Base Station from a Mac or wireless device that supports WPA, you will be asked to enter your WPA Personal password.
If you join the AirPort Extreme (802.11n) Base Station from a computer that supports WEP only, you will be asked to enter your WEP password.
Note: The password you enter will be the same for either computer. For Mac computers that support WPA it is normal behavior to be prompted for WPA, even though the Base Station is configured to use WEP.The AirPort Extreme 802.11n base station can be configured to allow clients to associate with the wireless network using WEP (TSN), several types of WPA, and WPA2. WPA offers a higher level of security than WEP, but some wireless devices only support WEP.
If you would like to verify that your AirPort Extreme 802.11n Base Station is using WPA or WEP, open AirPort Utility, select your AirPort Base Station, then click the AirPort Summary tab.