In your applications page in your "My Submissions" section you'll see a link under the "License Master Keys" heading titled "Create new master key pair". You should click on this link.

Once the link has been clicked on you will be asked to supply a name for the key pair and an API key. The name is for your reference, the API key will be needed to generate licenses, but don't worry if you forget your API key you will be able to get it from the system. Once you've entered this information click the "Create new master keys" page.