1. Following the spiders: He follows a giant spider and its web trail, which leads him to a secret passage behind a tapestried wall in the girls' bathroom.
2. Using the Basilisk's venom: After being bitten by the Basilisk, Harry uses the venom to write "open" on the wall, which reveals the entrance.
So, Harry doesn't use any specific words to unlock the Chamber, he uses his wit, bravery, and a little bit of magic to find the way in.