After trying to follow different instructions without success I found a workaround which works fine for me. (External disk connected via USB to my Airport Base station).
There is a hidden system preference that needs to be change. This can be done by typing the following command in the Terminal (in ONE(!) line).

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

After doing this the disk appeared in my network connections and was available to Time Machine without any further modification.

The initial kicked-off backup took about 2 days but now I have a constant wireless backup solution which works quite fast. This gives me additional security for small changes I make during a week - main backup is still done via firewire or usb disks connected solely and directly to my mac.