You DON'T activate. Legitimate companies do not send out checks prior to work being performed. You are being scammed and I will bet it follows the standard format--they sent a pfish email; you responded giving address, phone number, etc; they sent you a check and indication that you will be paid a large amount (like $100 per shop) to shop Walmart and probably Lowes and your final shop will be to wire transfer money by Western Union.
DO NOT DEPOSIT OR ATTEMPT TO CASH THE CHECK. When it bounces the bank will hold you responsible for covering the bounce and the bounced deposit fees. If you have wire transferred away the money already it is unrecoverable. It is one of those 'too good to be true' deals that indeed is not real but a scam.