Working on a character and when I try to select a feat in the character builder Dual Wielder does not show up as a option. 

That feat is in the PHB but not the Basic Rules. If you have not purchased the PHB, or at least the feats from it you will only have access to Grappler as that was the only feat published for free in the basic rules. 

Thanks for the answer. I know that I own all of the source books, it shows up in my feat list.

Do you already have that feat on that character?

Make sure you have one of your equipped weapons selected as Dual Wield ( click checkbox ON ) in the customize section for that weapon.

I have the players handbook but I don’t see any customization feature for the weapons is there any other reason the feat won’t show up?