I'm having a similar problem, but more complex than simply no users showing up.
At first, I had no users displayed.
Then after I unchecked the box, "Display users from trusted domains..."; the users showed up.
Then I tried adding a couple more users in the DC, and they didn't show up. I tried re-joining the domain with the box checked and unchecked, and specifying PDC and BDC both ways. The users still will not show up.
Here's the real kicker though. I deleted one of the users that did show up in the first batch, and it was removed from the ReadyNAS user list instantaneously. So I know it's capable of finding the domain users.
RAIDiator™ v3.01c1-p6 [1.00a140]
edit: Also want to add that even though the new users don't display in the list, I can still create a share and access it with that login. However I must have the users displayed in the ReadyNAS list so that I can enforce quotas.