There isn't an actual "Application Builder App". Instead, this corresponds to a set of customization features:

• The developer mode: adds a menu at the top of each page, that's let's you view and edit the internal configuration behind that page. It let's you change the View and even add custom fields on the fly. You can activate that in the "About Openerp" window (on the top right, click on the user name) you can find an "Activate the developer mode" link.
• The technical features: gives you access to all the technical configurations in the product, including others not available in the developer mode menu. You can activate in the admin user settings (Settings -> Users), by checking the "Technical features" box in the "Access Rights" tab. Refresh the page and in the Settings module you will see a new section "Technical".