I've messed around with this too, and had the same problem as you. The version of AFP required to connect to Airport disks or a Time Capsule is higher than Mac OS 9 supports.
The way around this to to use FTP. Turn on FTP sharing on Mac OS X machine, and then mount the Airport disk. Now fire up a FTP client on your Mac OS 9 machine. The Classic version of Transmit is free these days. Log in as the owner of the Mac OS X machine, so you have access to the root drive. Here you will find the Volumes folder. In there will be all disks, even networked ones!
This will work with any FTP client, so even a Mac Plus with Fetch can access an Airport Disk or TimeCapsule.
The way around this to to use FTP. Turn on FTP sharing on Mac OS X machine, and then mount the Airport disk. Now fire up a FTP client on your Mac OS 9 machine. The Classic version of Transmit is free these days. Log in as the owner of the Mac OS X machine, so you have access to the root drive. Here you will find the Volumes folder. In there will be all disks, even networked ones!
This will work with any FTP client, so even a Mac Plus with Fetch can access an Airport Disk or TimeCapsule.