Why is my usb hard drive not showing up in airport utillity 6 ?

Hey!

So I have just thought about filling the last plug in my airport express. I formated my HD to fat32 (I thought it'd work) and plugged it into my airport. Then I opened the utillity and...no "Disk" option. ....err, what?

What do I REALLY need to do, to share a HD with my Airport express?

Regards, Ingwie.