I was able to find this online even though I was at work. It may help those of us having trouble using the LFD tool to figure out why we can't use it.

The Dungeon Finder will try to queue you for ALL instances that you meet the level/gear requirements for, and if you can't enter one or more of them due to having not found the entrance it will fail. So go find the entrances to the rest of the instances you're eligible for.

So in my case I was likely eligible based on gear level for multiple dungeons but since I hadn't found them yet it was failing on the dungeon check. Once I go and find those dungeon entrances I am guessing I should be all set.

TLDR version: As long as you have discovered all of the dungeons that you meet the gear requirement for then you should be able to to use the LFD tool