To install Workflow for Professional Edition, you must first enable Ideas in your Salesforce Org. To enable Ideas please do the following: 

  1. Go to Setup >
  2. Customize >
  3. Ideas >
  4. Settings >
  5. "Edit" >
  6. "Enable Ideas", then click on the very first checkbox,
  7. "Save"

Return to the AppExchange and initiate your install by again clicking on "Get It Now.