Both your API key and your secret key can be found at https://publishers.viglink.com/publisher/sites . Click on the "key" icon to reveal the keys for each of your sites:
This will reveal a page with your site's API key and an option to generate a secret key (which will allow you to use some of our API tools):
Note: When using the secret key for API tools, remember to use the word "secret" along with your key. That is, "secret 1234" rather than just "1234".