As a Page Tab App developer, you can prompt users to install your Page Tab App by including an “Install this Page Tab App” link that explicitly directs the user to the /dialog/pagetab
endpoint, either within your Page Tab App or Website:
http://www.facebook.com/dialog/pagetab?app_id=YOUR_APP_ID&next=YOUR_URL
where YOUR_APP_ID and YOUR_URL can be found in your app settings.
This URL brings up the Add to Page Dialog:
Read more in the documentation.