Guys, if you just TL;DR all the quotes from the Wiki, you're never going to learn anything.
As posted earlier in the thread your target must meet the following criteria:
-Level 4 or below.
-Disabled by Paralyze or Sleep.If they meet BOTH of those, a PC of any class or level may attempt an attack on them with an automatic roll (to guarantee a hit) that does not deal damage, but rather kills the target outright without a roll.
Assassins have a Death Attack, which is something else entirely that just paralyzes an enemy upon a failed Fort save. In theory, if that enemy was below level 4 and they somehow survived the initial damage from the death attack - yes, you could in theory perform a Coup de Grace. But it has nothing to do with, and bears no relation to, the Assassin Class or class abilities. It's the same as if a Wizard were to cast Hold Person and then run in to Coup de Grace with a dagger.
The reason it's not working on the Ogres in the Forest Fane is, of course, because they are too high level to m eet the criteria for Coup de Grace.
Edit - Source -
http://nwn.wikia.com/wiki/Coup_de_gr%C3%A2ce