The API key is sent immediately after you have signed up to the email address you entered. Sometimes, the email can get caught in a spam filter somewhere along the way. Worry not and log into your account. From there you can generate a new API key by clicking the “Generate a new API key” button.
Your previous key will stop working and a new one will be shown for you to copy and use.
Important: please allow 5 minutes for your new key to start working!