you just have to download both MC and MC cheats plus the hybrid mod. put everything on your mods folder.
ex: Documents\Electronic Arts\The Sims 3\Mods\Packages
and they will appear in-game. usually there will be a notification before loading your save file of the list of mods you have once you get past through the loading screen. try looking in the list to see if the mod is listed to know if it was installed correctly.
Now in game click on a sim of your choice that you want to have a hybrid gene. look for Nraas>Master Controller and a window will appear. now go to Intermediate>Occult: Add
you can also use elixir's to make hyrbids. like throwing a Bottled Witches' Brew to a fairy or vampire.