If it's an outside wall it's a real b!tc# because of insulation.If it is you can save yourself a lot of drama by just taking the wire thru the attic,drill it to the outside,take down the wall outside(you can staple it when your done),then drill it back inside where you want it.If you have a crawlspace that is a much better option.
On an inside wall,if there is a power outlet already there use it to get your bearings from the attic.Odds are it drops down the wall from the attic so you should see it up there.I don't recommend using the same hole/path as the electrical but use it for reference.Go about 2 feet on one side or the other & drill down thru the studs.Down below,measure the same distance from the electrcal outlet as you did up in the attic.It helps to find the studs before you do all this,by the way.Then cut about a 2" X 2" hole in the wall.Drop the wire down in the wall from the attic & have someone try to snag it with a bent coat hanger or something like it.You will need help with this because the wire rarely appears right in front of the hole in the wall.You'll need to stay in the attic while they try to snag the wire.When you're finished a standard CBTV outlet cover or RCA stereo plug outlet will cover the hole.
Once again,if a crawl space is an option go that route.Good luck.