My group and I have been arguing this for a couple hours now. The stacking of IUD and uncanny. The level 10 ninja in our group wants to take a level in shadowdancer. He says because he already has IUD from his ninja he want be flanked by a level 14 or lower. I have argued he can't stack his uncanny dodge bonus from the shadow dancer because he has not achieved that ability from his shadowdancer yet. The rules seem a little vague to us so I was wondering if there was an errata or somethung
It is not vague; you are correct.
Improved Uncanny Dodge (Ex): At 8th level, a ninja can no longer be flanked. This defense denies another ninja (or rogue) the ability to sneak attack the ninja by flanking her, unless the attacker has at least four more ninja (or rogue) levels than the target does.
If a character already has uncanny dodge from another class, the levels from the classes that grant uncanny dodge stack when determining the minimum ninja (or rogue) level required to flank the character.
That (and the same clause in Shadowdancer) is the one and only clause that allows level stacking, and it clearly says he has to actually have uncanny dodge from another class in order to stack the levels from that class.
If you like, you can cite this FAQ:
When do I count as having a class feature?
You have a class feature when your class description tells you you gain that class feature, generally based on your level in that class (and perhaps altered by factors, see below). [...]
Until he reaches the shadowdancer level where uncanny dodge kicks in, he doesn't have the uncanny dodge class feature from shadowdancer.