Open the main entrance is a mission in Going Commando that takes place in the Silver City, Boldan. It occurs concurrently with "Find Mr. Fizzwidget", and is a prerequisite to continue. While Ratchet and Clank traveled to Silver City to find Abercrombie Fizzwidget, they found the entrance by the ship completely blocked. The two resolved to take a taxi to the hyper-expressway, and search it using the Levitator.
This mission involves no combat, and solely makes use of the Levitator. After taking a taxi to the hyper-expressway, take the elevator shaft down to a large tunnel filled with cogs and machinery. Use the Levitator pad to fly across to the end of the tunnel, and into a platform in the middle of the expressway, making sure you avoid the cogs that slow you down. Use the Levitator pad on the next platform to fly against the traffic through the tunnel to a ledge in the wall with another Levitator pad. Finally, use this Levitator across the next tunnel, dodging the electrified cables, and you will reach an elevator that goes up to the inside of Silver City. Once here, stand on the platform to open the main entrance, allowing you to continue the missions "Find Mr. Fizzwidget" and "Explore Silver City".