The first thing you'll need to do is connect your Campaign Monitor Account to Zapier. After you sign up and start creating the Zap you want, Zapier will ask you to start the process:
You can then give your Account a title or label, which is just for your convenience and helps you track multiple accounts. It doesn't have any affect on which Account you actually add to Zapier.
You will also need to enter your API key, which can be found at https://YOURSUBDOMAINHERE.createsend.com/admin/account/
Finally, the Account will be added and tested, if everything is good to go you'll see this:
Now you can continue creating your Zap.