To add a combination to a course, you need to have an account and be logged in. If you haven't got an account, you can register for free. Next, you need to find the competition you want to enter. You can search by using the search bar at the top of each page.
Once you've find the competition, you'll get an overview of it's courses. Click on a course to view all the subscriptions. If the course isn't full or closed, you'll see an 'add' link next to each start number. Click on it to add yourself.