Hello.
I have tried this addon today, but nameplate scanning (which I am/was under impression, that it scans nameplates displayed on the screen, even if mobs are so far that I can hardly read the name) doesn't seem to work. All rare mobs I've found today were registered by the addon only on mouse-over (with default "I see you" alert). Even when I was next to a King Ping (Northrend, Howling Fjord), who was attacking me, then only when I have targeted him was alert triggered (probably also mouse-over).
I was going through Northrend so far. I am starting playing, so I don't have any other addons.
Attaching debug output.
silverdragon.txt

There's two things you're referring to as "nameplates" here -- the ones that're far away are when you just see the text of the name, and the things Blizzard calls nameplates which is when there's also a health bar shown. The latter ones only appear when you're fairly close to the mob.

King Ping, in that log, was seen by the nameplate, but you were in combat so it couldn't show the popup until later.

I was in a combat with him, because he attacked me (I was avoiding moving mouse over him). So there is no way to be alerted to rare creatures when I have them in sight, but I didn't notice them among other creatures?