How-to: Setup multi-Mac backup shares with quotas. Easy.

I previously used these instructions to set up Time Machine for my girlfriend's macbook successfully, then she updated the OS on her macbook from Lion to Mountain Lion and now when you open up the Time Machine preferences and Select Disk, the NAS doesn't appear as an option. I re-checked that I followed the steps correctly from the initial post and that the necessary files were correctly configured on the NAS for it to work and they were, just to be sure, I deleted and re-created her share and went through the steps again but the NAS was still not visible as a disk to backup to.

In the meantime, I need to start using the ReadyNAS Ultra 2 to backup from my macbook using Time Machine as well as I'm selling the external hard drive I am currently using. On my macbook I am running Lion 10.7.5; again, in Time Machine preferences, the NAS does not appear as a disk option to back up to.

In both cases, we are connecting using an ethernet cable to connect to the NAS as opposed to using the wireless.
Using RAIDiator 4.2.22.

Any suggestions on how to troubleshoot this as I really need to get the NAS operating as Time Machine backups for both our macs as soon as possible.
Thanks in advance.

**UPDATE**
I re-booted the NAS and re-started the router.
Not sure if it's related to doing either but the NAS began to show up in the Select Disk option in Time Machine preferences... great! However before selecting it to start my backup, I wanted to start backing my g/f's macbook first but it didn't show up in her Time Machine prefs. Now it has disappeared from my TM prefs...
Not sure what is causing it to visible/not visible - the NAS is visible and am able to connect via Finder the whole time on both macbooks.