First you'll need to make sure that the network volume you want to use is mounted first. Then you should be able to just navigate to whatever folder you're planning on using as your new home directory and select it. Network volumes in the OS X Finder can typically be found in the "Network" item under the main directory of your computer (in the Finder, you can hit ⇧⌘K to go there directly). The absolute path if you're using the terminal is typically /Volumes/Share Name.
It's worth noting that if you login without the network drive mounted, things could easily break. For this reason you'll want to make sure that you've set up OSX to mount the share on boot (prior to login). This doesn't happen with SMB (Windows file sharing) or AFP automatically, but there may be a way to hack it, maybe others can chime in with this part. NFS shares set up through Disk Utility mount automatically, but I'm not certain when in the boot process they do, I believe it's pre-login, but I can't find any information to corroborate this.