At the start of each level you will see a briefing that says how much Rockets, Fuel and Armor you must collect. On the right you will see four indicators with the colored icons, they will remind you how many resources must be collected.
Click "Proceed", and you will see the Manipulator and many things on the floor. Collect them into the box floating behind.
Use Left/Right arrows to move the claw sideways, Down arrow to move it down and Up to grab/release the resources.