On my W-4 I added $120 to line 6 for additional amount withheld. I am wondering how I go about collecting the money that had been withheld all year?

[BobinCT]

You get credit for all the withholding that was taken.  So if you added $120 you get $120 more credit against your actual taxes and if you were getting a refund anyway you will get a $120 greater refund, etc.