You can never really 'use' them in a sense of building them into designs, because that would require you to have the technology entry, in which case you also could just build them yourself.
You can however use them to research the background engine technology. To do this, just unload the components on a planet where you want the research to be done. It is important to chose the right planet, since every generated tech points will only stay on this planet and no other will benefit from the boost until you successfully reach a new tech level in the area.
To get those tech points, head to the building window of the planet in question, click to show components/missiles/fighters and such, and then select the component you want to be researched and press the disassemble button. Your message log will then (aside from resources) show you a couple of tech points that had been harvested in a corresponding area. You will need many many components to make the jump to inertial fusion however.
A tip: For rapid tech advance, your best bet is capturing enemy ships instead of destroying and hoping for undamaged loot. You can repair the captured vessels, then take them apart and instantly gain most of their components. To capture, send marines(-best choice) from drop modules to land on an enemy ship. Then wait many minutes until they are done.
