Go to page you added the app to (this link takes you to all your pages). Click More below your Page's cover photo. Now click on the tab that says Welcome.
If you still can't find it, Make sure you’re logged in as yourself and not as your page when editing page apps. To do this, at the very top right of your Facebook to the left of the lock icon, click on the dark blue down arrow. The select your name. Facebook instructions are here.
If you want to feature your tab under the cover photo follow this tutorial: http://screencast.com/t/EGvUAMRnv