For each online service you have registered for, you'll get a separate Activation Code sent from the Government Gateway. Your CIS Activation Code will be sent by post to the latest address that HMRC holds for the relevant PAYE scheme. You must use this code to activate the CIS Online service within 28 days of the date on the activation letter.
Once you've received your Activation Code for your CIS scheme Log in using your User ID and password. Select the CIS service you have registered for and you'll be prompted to enter your Activation Code.