Your API key can be found by logging into your account and going to the Account Settings page.
You'll see an option to Show API Key, as pictured in this example:
Important: The Show API Key option is not enabled until you have verified your account. When you first sign up to Campaign Monitor, we send you an email with a verification link in it. You need to click that first to verify your account.
For information on developing your own applications that integrate with Campaign Monitor, view our API documentation.
Remember, your API key is like a key to your account, and all its data, so only use it in 3rd-party websites and applications that you trust.
You may also consider authenticating using OAuth rather than an API Key.