There are two easy ways of Using a Network Drive for TimeMachine backups:
Way 1:
Create a growing Diskimage on the Network Drive, mount it, and you should be able to select it as Destination in the TM PrefrencePane
Way 2:
Execute
CODE
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
in Terminal, log off, log in, select the Network Drive in the PreferencePane, be happy

I'm using the second way to back up my MacBooks HDD on a samba share (maxtor shared storage II) over WLAN.
Greetings,
bundeswerner