1.: Did STEAM already download the newest patch for C&C 3: Tiberium Wars? If not update the game.
2.: Do you have a "Mods"-folder in the STEAM-directory of "C&C3: Tiberium Wars" or inside your "C&C3: Tiberium Wars" folder inside of your documents folder? IF YES GO TO STEP 3. If no create a new folder called "Mods" inside the folder called C&C3: Tiberium Wars. You have search for this folder I think, don't create it yourself inside STEAM because I don't know how STEAM arranges the "C&C3: Tiberium Wars" data.
3.: Make there a new folder with the Name of the mod, in our case "Tiberium Essence" and then you put inside this folder all data from the downloaded ZIP-file for Tiberium Essence.
4.: If you don't know how to open the C&C-startup center you can modify your steam shortcut for "C&C3: Tiberium Wars" inside the STEAM library. Just right click on it (in the steam menu) click "properties" and then "start up options" or "starting options" or "start options", something like this and enter "-ui" and click ok, I think then it should work.
5.: Start the C&C-startup center, go to "Mods" and choose "Tiberium Essence".
I hope I could help you.