How do you get the man in green to be in the pokemarts?

That is done by transfering Pokemon over from games other than GBA poke ones and Diamond or Pearl.
For example he appears when you transfer a manaphy egg over from ranger.
He only appears in the Hearthome poke mart though

Those guys in green suits only appear when you access events in the game. A couple examples include: the Arceus, Darkrai, and Shaymin Events. There's just a couple more events you can go through on the game. If and when these guys appear, they will give you a certain item that helps trigger each event. For example, the Darkrai event. You would get an item from them called the Member's Card. Take that to the locked building in Canalave City. Thus, triggering the Darkrai event. You can access these events when using certain codes with the Action Replay.

Hope this helps,
Sk8rboy

You get that guy in the green suits by getting a mystery gift and accesing an event or if you don't want to wait for an event get an event code for each of the following (arceus,shaymin,or darkrai) or if you want to open the inn at canalave city you get the code that gives you a members card