Go to System Preferences
Highlight AirPort and click Configure or Advanced depending on version of OSX...
Choose “By default, join: Preferred networks”
Select your access point and Remove your access point with the minus button.
Launch your keychain access in Utilities and delete your access point keychain entry.
Go back to the “By default, join:” page and click the plus this time to add your access point. Enter the correct password, save, reboot.
Instead of joining it from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.