@lilyhenderson1 When you state to have MCCC installed you should be able to check it by switching on the options in the game menu. First look for other in 'Game options'. There you wil see the options for 'enable custom content and mods' and below that 'script mods allowed'. When you tick these boxes your mods will be enabled in the game.
As soon as you start a game and click one of your Sims there will also be a choice which says 'MC command center'. When you see this option you MCCC will be enabled.
A word of advice: using Mods is not without risks. It could damage your games and saves. MCCC is a very powerful mod and should be handled with great care. If you don't feel up to it. Just don't use it.
Have fun playing. 😉