CAN SOMEONE HELP ME WiTH MY SINNOH DEX?!?

i saw every pokemon accept cressila and heatran for heatran you need to go to stark mountain right? if so how are you supposed to get there if those 2 people are standing in your way waiting for you to complete your sinnoh dex obviously you cant get p to go to stark mountain to see heatran if you havnt seen heatran?