Hi,

I'm trying to put together a homebrew feat that for one round, allows someone to double their proficiency bonus on a melee attack.

Selecting the "Twice Proficiency", or "Expertise" modifier types don't change my characters attack bonus at all.

Is there any other information needed for the Twice Proficiency modifier type?

The twice proficiency modifier only affects skills. There is no modifier for doubling proficiency for attacks. You can choose a variety of modifiers for adding set bonuses to attacks but none that can be based off proficiency. 

Also, the character sheet isn't really set up for modifiers applying on "temporary" things, generally if you add a modifier to, say, an item, and then equip the item it applies the effect permanently until the item is removed. This can be cumbersome for 1-round effects. It would be easier to just say in the game "I'm using this" and just add the proficiency bonus  (displayed nice and big on the character sheet already) to the rolls already indicated. You just mentally add +2 to +6 depending on level. 

Unfortunately, the Homebrew creator leads users to think otherwise. It gives you a whole list of suboptions under "Twice Proficiency".