Unfortunately, you need to have the National dex, and for that to get the National DExto happen, you need to have seen every Pokemon in the Sinnoh dex. After you see every Pokemon, go to Professor Rowan in Sandgem Town, and he'll give you the National dex. Right after, Professor Oak walks in, and allows you to find him at his temporary home in Eterna City, where you can the trac the Legendary Bird Trio, but that's another thing entirely. After that, go ahead to Snowpoint Temple, and Candice will give permission for you to go in. There is, at the bottom, a sealed Regigigas who can only be unlocked by the legendary titan trio, which requires the Regigigas from the Pokemon 13th Movie distribution.
EDIT: I just read your question a bit more, and here's your answer: As you have already seen every Pokemon in the Sinnoh Dex, you just need to talk to Professor Rowan in Sandgem Town to get the National Dex you need to get into the temple.