It appears that a lot of people have problems connecting to a wireless network with WEP security when using AirPort, as was my friend. It also appears that hardly anybody knows the fairly simple solution, so here goes.
First of all, don’t type your password in regular ASCII text, but type the hexadecimal value (you can translate between the two here). But that’s not all, you have to also add ‘0x’ in front of the password! So let’s say your password is ‘mario’ then in the AirPort settings you type the following password: 0×6D6172696F and you should be able to connect.

If you’ve encountered this problem and found this solution to work, please share it so everybody will know what to try when they get that magnificently detailed message: “There was an error joining the AirPort network”