To reach the Mausoleum of Origins, you need to talk to the archeologist in Fuchsia City or saffron city, in the tent directly right of the PokeCenter.
The archeologist will tell you that he needs strong trainers to help him explore the Mausoleum, and challenges you to a battle.
He has six Pokémon, the first three of which are Pokémon that can only be found at the Mausoleum, and the last three are all legenderys.(regice,registeel,regirock)
Upon defeat, he will take you to the Mausoleum. To get back, you can use the Taxi NPC
Details
The Mausoleum of Origins is the only location for many Pokémon. (About 20-30+) This includes the Legendary Zekrom and Reshiram.
Some Pokémon are only found in grass patches inside the Mausoleum, while others are only found With a Kyurem in your party.(Zekrom and Reshiram).
This is also home to the Musician, which allows you to battle/catch Meloetta.