It's in the Lapis Ward, so you're at the right place but you have to solve the magma/aqua storyline and i don't really know but i think you have to beat the psychic gym leader to get into 7th street... so you need 8 badges
I don't know if it's just me or if anyone has the same problem.. but in my situation the bouffalant doesn't destroy the first log on the way to the place where you find heracross/pinsir (2nd picture in vinny's spoiler)
I don't know what to do