I'm pretty sure you don't need a certain expansion to use it. It just unlocks at 15. It's especially wonderful for healing and tanking classes who can't/won't solo. Since it can be kinda rough. They're able to just queue up and use it all the way to 85 if they really wanted to. Surprised you haven't used it so far. How have you been getting groups for instances? /boggle
Maybe I just said "LFG tool" instead of "Dungeon Finder" and it threw the both of us off? Probably the case.
But I think from 15-60 it rewards with a goodie bag that has a reasonable chance to have a useful blue item in it. Gear, that is. Boots, bracers, rings, etc etc. After 60 it starts to also give badges. And with these badges you purchase gear. Works as a currency. Its kinda for those who never win rolls on gear in an instance, but can still save up badges and purchase some gear regardless. Now, I could be wrong, but they merged all the badges together as just the 1 type. Since there used to be a lot of them and it was not only confusing, but also made your unused old-expansion-gained badges pretty much useless. So it should be safe to run all the BC instances and just collect badges and gold from using the LFG tool. Then once you have enough saved, you find the vendor selling the gear/items and buy what you want.
And like I said earlier, I think the vendors you want are gonna be on the Isle of Quel'Danas. Which, I don't know if you've been there or not. But in Shattrath, there is a portal to it in the center ring of the city. You can take it to get there, but to get back, you'd have to fly do the nearest city, find the portal trainer, take the portal to Blasted Lands, go through the Dark Portal, and then fly back to Shattrath. There's also a quest reward for one of the dailies that allows you to hearth back to Shatt, as a one-time-use.