First, airmon-ng is there to put your card in monitor mode. You probably meant airodump-ng. (I assume.) Second, if this is wpa, then just listen anyway to get the handshake. the client will be included in there, but you have to use a separate program to get it out for you, i think. I suggest elcomsoft distributed password cracker. You don't need to crack the password, the client mac will be displayed anyway. If this is WEP then theres a tool that'll do it for you.
I looked through the script and im pretty sure this will do it. If not, just edit the loop to sleep and check for clients every once in a while.
Gotta love these forums. Everyone has already done everything.