Enter the domain you'd like to generate keys for, for example dkimcore.org and hit the button.
This online wizard is fine for generating keys for testing and evaluation. If you decide to use DKIM Core in production, though, you might want to consider the security risks. The wizard records your DKIM Core keys, including your private key, until you delete them.
That means that I have access to your private key, and could forge email to appear to be from you. I'm not going to do that, of course, but if you're concerned about the risk then you can generate DKIM Core keys on your own machine using openssl, as described in the specification.