Tab "Applications" is not available in customer's control panel.
Make sure that access to APS catalog is allowed on service plan of subscription. If subscription is custom or locked, check allowed permissions at Subscriptions > subscription > Customize > Permissions tab.
For Plesk versions prior to 10.4: verify that option "Application Vault" is enabled on Plesk license (Tools & Utilities > License Management).
Check that APS has not been intentionally disabled in
panel.ini(see KB #112766)