How to build fusion reactor?
I just started on working on documentation, so there is no up to date wiki for the GT5U fusion reactor, but:
Use the http://ftb.gamepedia.com/Fusion_Reactor_(GregTech) and http://ftbwiki.org/Fusion_Reactor_Mk2_(GregTech) for GT4.
Instead of fusion coils and advanced machine casings use the blocks discribed in the fusion computer tooltip.
replace energy injectors with energy hatches,
material injectors with input hatches and
material extractos with output hatches.