What prerequisites are required to start this quest where Baldor Iron-Shaper(blacksmith of Skaal village) goes missing? I've been craving a Stahlrim bow.

UESP hasn't been of any help, and there hasn't been an article yet on the official wiki.

im pretty sure the main quest has to be completed...cause after the shaman dies all they say is "i hope it was worth it" and "you better kill him" bla bla bla.....then you kill him go back to the village and talk to them...all of a sudden hes missing

Incorrect.

You only need to finish "Cleaning the Stones". Especially the one near Skaal Village, and the NPC's must survive the lurker attack. Leave the cell (Like go to Raven Rock), then travel to the Village.
Eventually there will be a dialog about someone going missing. Pick that quest up, and you're on your way.

Here's a link:
http://uesp.net/wiki/Dragonborn:A_New_Source_of_Stalhrim

I've finished Cleansing the Stones, and come back to Skaal village multiple times since then. I'll have to check the stone near Skaal village, then.

(After my brother gets off Assassin's Creed 3)

Thanks for the help anyway.

Define "Multiple". It took me about two returns before the dialog happened. There's also the factor that actual time might need to pass (Days, perhaps?)

Also, part of the main quest will block off dialog from -all of them-, up to beating the game. BOO was only half wrong on that account.

I travelled back to Skyrim before it popped up for me, though that seems to reset cells every time. Or at least my house does.
Also, there is the possibility of glitching yourself out of the quest if you speak to them (Deor and Morwen I believe) while they are talking about it.
When you fast travel to Skall village they are standing right at the entrance where you spawn, maybe you happened to try talking to them as soon as you spawned without realising?