I have seen a lot of people asking about how to get in. I struggled too. What I found, by reading around, is you have to do stuff around town. So, here is a log of what I did.

1) Visit every building available
2) Obtain the Ralts
3)Beat the battle restaurants.
.
.
.
4) Profit?

I highly doubt number one is necessary but I think 2 and 3 are. A lot of people seemed to have missed the ralts pickup.