When you start creating a Act-On Zap, you will be prompted to connect a New Account:
Click the button and a pop-up would appear. You would then be prompted to authorize Zapier to allow access to your Act-On account
You would be then asked to enter your Act-On Username and Password:
Once you've entered the correct API key details, the pop-up would disappear and you will see the Act-On account added to your Zapier account and is ready for selection.