Assuming you have the Foundation installed, it's under options.
If you also have UnifiedMainMenu (UMM) installed, then you first need to activate the Master Switch under Customize, and enable the Mutators under one of the menu's on the same level as the Master Switch. After you have enabled that, you need to reload the options by, for example, first going to New Game and then back to Options.
If you don't have Foundation installed, then you should, since it's the, well, Foundation for all the mods since a very long time. You can install the latest through BCUT (found on BCFiles.com), if a mod asks you for BCMP or BCMI(.Net), then please use BCUT, since it's newer and less error prone.