Start by following the registration guide, it could be a good idea to use the same username as on geocaching.com. Note that your user will not be created until you confirm your email address by typing the code you receive.
Before you are able to use cachetur.no you need to authenticate against your geocaching.com-account.
Press the blue «authenticate»-button.
You will then be brought to geocaching.com, where you have to log in, if you're not already. After you have logged in, or if you're already logged in, you'll be brought to the authentication page.
Press Allow Access to continue. You are now authenticated against cachetur.no, and will be forwarded to your dashboard.