No. Every employer account takes part by default, from the day the program started counting for it. You don't have to switch anything on, and nothing about how you post jobs or review applicants changes.
Earnings build up on your My earnings page before you do any paperwork. The terms, the W-9 and your payout details are only needed when you want to be paid — the page walks you through them under Getting paid.
Payments made before the program started counting for your account don't earn: the program only ever counts forward, never back.
If you'd rather not take part, see "Leaving the program".
Your My earnings page shows every payment that earned, what is clearing and what you can be paid. Reply to any program e-mail and a person will look into anything that looks wrong.