It needs to be active at the end of the battle, every digimon has their own skill which only activates when they're in the battle. The best way to get platinum numemon is via gold numemon since he can warp digivolve into him. Platinum sukamon does the same thing but he's not a mega and holds less items - tactician USB's can be developed on the farm which stack with plat numemon/sukamons effects. Google it and you'll probably get a ton of results, just remember to keep a team around the same level as what you have now and slowly level them up, so the game doesn't get boring.
I follow your advice, thanks!, and I got 3 shinny metal poops =D, also I got 2 tacticians USB for now, I also got p numemon, but it uses to much ram and I don't have too many tacticias USB for now, anyway I'm using 2 sukamons at the same time because with 3 the fights gets too long.
Anyway, I already got Mastemon(I think I regret this one, I was happier with ladyDevimon and angewomon), Imperialdramon, and one more master that I don't remember now. I'm going for Lucemon now, but I think I will continue a little with the history, too much grinding for now. I'm in chapter 18 or something.