Putting engine into mech

[Terkala]

I tried dragging the engine into the center of the mech, but it wouldn't go. I couldn't figure out how to get anything to work.

[KiberKreker]

First you need to assemble and turn on the engine.
In the basic component version, engine off is displayed.
More in game hints and advice will be on release.
Good luck.

[Nathan the Dwarven Shark]

In the Mech Assembly screen (where you can attach modules and weapons to the mech) drag the reactor to the bottom-centre, the area you can see the preview of the currently selected component. This will then allow you to assemble and test the generator. Upon tweaking the Reactor to your liking, ignite it and place it to the left (and repeat with other generators). This activates the reactors and allows them to be mounted.