In the mans house on mossdeep city there is that door in the back. How..

Getting through that door has something to do with an E-Reader.

Either you use a cheat device or an e-reader to know what's inside that bloody door.

There's not a key! If you have an e-reader and a 2nd GBA or GBAsp (or borrow one) and a link cable, you can get 3 packs of trainer cards and super rare berries that allow you to "scan" them in. If you scan in a trainer, he will be behind that door for you to battle!
If you don't have the set-up, you can get all the necessary items used at GameStop/EB Games (an old GBA used, e-reader used, & link cable's are cheap enough new or used), and they are one of the only places you can find those cards since they are only for R/S. Good news, they're only about $1.50 a pack, because they're on clearance. If you're local store doesn't have them, ask them to call other stores or try their website-I know they're happy to get rid of them! They are called Pokemon Battle e-Cards, and the 3 packs are: "Hidden Ruins", "Iron Defense", and "Freezing Ray". Another cool thing, the trainer's Pokemon hold some very good items, so be prepared to steal them away for extra goodies for yourself...it tells you some of the info on the card.
This is the complete and correct answer, trust me, as I have them all. A walk through walls cheat will not get you anything, and may mess up your game!! And in other games, when you see a similar door, that's why it's there, but they did NOT release e-cards anywhere but Japan, so before you even ask, no, there's nothing you can do in FR/LG or Emerald as of this writing with that door or behind boxes. Altering Cave is the same thing, no cards for us; but in Japan, yes.