How do you go in to the Pokemon mansion in Pokemon Platinum?

you walk into the front door... but if you mean the maids after you get the national pokedex run down the hall on the left until the maid stops you. talk to her, and you will battle five maidds in a row. you have to beat them all in one turn. I'm not sure what happens after that though, but i assume you'll at least be able to enter the door that the maid in the left hall was blocking. i don't know about the maid all the way on the right though...

you go south and find a gate

when you beat the maids and get to correct amount of rounds , the lady of the house will come to battle you with blisim (correct me)
You Can Find the mansion on route 212. When you just enter Heartrome on the south but some people are blocking so you need to go the other way onto pastoria city to go there once you beat the pastoria city gym you can go the heartrome way