First Navigate to the Plugins page of WordPress and click "Add New"
Download the Zip File that is attached to this support doc. Then Click the Upload button
Select the Zip File that you downloaded and Click Install
Once you activate it you need to create your PayWhirl account(or just login). Click on the new PayWhirl plugin (or go to paywhirl.com)
Once you have an account created you will end up on the PayWhirl Dashboard. This is the main screen behind PayWhirl. Right now we need to complete the integration by registering the "webhooks". This will allow you to easily track and monitor orders.
Now you need to fill in the URL of your website. Make sure it ends with the "/paywhirl-webhook"
Now you can return to the dashboard and start making your plans and then when your ready you can embed your widget.
See Installing a Widget in WordPress