If it's the one all the way at the top level...you have to open the power/fuse boxes on every level to get the door to open at the top and the coin is inside...I know for sure the first one is on the ground floor on one of the support beams holding the rocketship..
Be aware that on every level of the rocketship, there are SEVERAL power boxes, one of which is for the coin, and others for green lasers and stuff...so make sure you get every single box on every level so that way you know for sure you didn't miss any.
Hope this helped.