In short, if you want to join the dungeon finder queue for level 100 heroic dungeons, you will need to get a silver medal in the proving grounds for the role you want to queue for. That means if you want to heal, your DPS silver medal isn't good enough. You'll need to go back and get it for healing as well. This applies only to the random queue. If you're going straight in with friends, no medal is required. Normal dungeons will not require any proving grounds experience at all, and normal dungeon and scenario gear should be enough to let you queue for the raid finder.