it should automaticly I think, but I saw some thread somewhere from the official forums, cant find it now but heres the gist of it:
Add one of these lines your Config.WTF file:
For a Dual Core Processor:
SET processAffinityMask "3"
For a Quad Core Processor:
SET processAffinityMask "15"
Doesnt show that much gain if I recall right, havent played in a while.
Think this just allowed it to thread to more cores, but there isnt that much threaded in the game to make any serious use of a multicore setup or something.
Ah, there we are:
You can use this to let WoW run on a specific core(s) but it only works up to two cores. If you set it on 15, you just let the game use 2 out of your 4 processors but you didn't tell it which ones.
Again, you must have MORE THAN TWO logical processors. You can see how many WoW has detected by looking at the coresDetected variable. For example, in my computer which has one physical DualCore processor with HyperThreading enabled, the line reads
SET coresDetected = "4"