When you start creating a Pardot, you will be asked to connect your Pardot account.
Note, Pardot API access is only included with Professional and Ultimate edition accounts of Pardot. More info can be found on Pardot's page here
Next, you'll be asked to enter your Pardot Email, Password and API key.
To locate your API Key in your Pardot account, visit the account's setting page and select the API User Key
Paste your key back into your Zapier account and then “Continue”.
If all steps were successful your Pardot account will now be successfully connected.