for MCC you can use it to change relationships and the rate a sim increases skill. to change the relationship between sims you go to MCC >> go to active relationship (make sure your sim is actually talking to the sim you want to change the relationship with) >> then you'll be asked to alter the friendship or romantic relationship and depending on what you choose you'll get different options.
and then for skill you do MCC >> flag active sim >> MC Skills and this will change how fast your sim gains skill.