How can you play as the the eclipse in empire at war forces of corruption?

The only way you can play as the Eclipse is on the Kuat level of the campaign. However you can download mods and addons to be able to use the Eclipse. For example z3r0x's FoC Addon allows the use of many ships not originally in the game. just visit http://empireatwar.filefront.com and in the search box type z3r0x and you will be able to download it to play the Eclipse in Galactic Conquest.