All you need to do is log in to My Account and from your dashboard choose 'My Bookings'.
Click on the 'Add Booking'.
Enter in the Thomas Cook booking reference. you'll find this on your invoice/confirmation and your surname.
You'll then receive an verification email with a link on to the email address you booked your holiday with (just in case this is different to the email address you created your account with).
Activate the link in the email and your holiday will be added to your account.
Remember you can add holidays you have already been on as well as any future holidays. You can only have the booking attached to one account.