When attacking, do you add your proficiency bonus to the attack if you're proficient with the weapon you're using? Or does being proficient with a weapon simply allow you to use that weapon without suffering disadvantage?
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The former. In this edition, whether it be a skill or a weapon, as long as you're trained in it you get the proficiency bonus. Other feats, class features, etc. may add to this afterwards.
"Proficient in a weapon? Add your proficiency bonus to your attacks with that weapon." from the source: http://www.wizards.com/dnd/Article.aspx?x=dnd/dndqa/20130919
So basically it means if you are proficient you can add your bonus if not you can use it but without the bonus
Not one of them, but both.
Actually, it works both way RAW. You are stripped of your proficiency bonus and disadvantage is imposed on attack rolls if you are not proficient with the weapon. According to the NDA, I am not supposed to disclose the actual paragraphs, so I would instead say that you must look into "Attack Basics" section in How to Play part, and "Weapon Proficiency" section in Equipment part. With both, the conclusion is that it works both way.
Basically, your attack roll looks like d20 + ability modifier + weapon training (if any) + situational modifiers. RAI, you can say that your "proficiency bonus" substitutes for your weapon training, you get no such bonus because you are not trained in that weapon, and even worse, you don't know how to use that weapon at all, so you are further penalized with disadvantage.