depends on which one you're referring to.
if it's the one in step two, it's just to the left of the big sign showing a map of the sub. you need to break the ground over to the left of that, get the remote for the floating platform, go back to the beginning and get the platform, and bring it back to the map. be sure to lower the platform everytime a torpedo passes, or else it will get destroyed and you'll have to get a new platform from the beginning. once the platform is in place, use slow to get on it, then use mach to raise it to the next level.
if it's the one in stop four, it's either right before or right after the section where the torpedos come out of the ground (i forget which). you'll see a section of the floor you can jump down (duck + jump). go down several levels (collecting the vfx canisters along the way, if you wish), until you get to the bottom. keep following the tunnel, and eventually you'll fun into some gears you have to stop, and then you know you're headed in the right direction. follow the tunnel til you get to the end.