Speak with High Explorer Magellas in Ironforge.
Well [player]... we seem to be at a temporary dead end here. We know what we need - the Plates of Uldum - but we have no idea where they might be. We'll need to do quite a bit of research on this to come up with a solid plan of attack. I have a couple ideas where to start looking; I have high hopes that soon, we'll be asking you to head out and recover them.
The High Explorer needs to be informed on this, [player]; no doubt, he also wants to thank you personally for once again aiding the League.