Updated for OnApp 5.2 (7th December 2016)
At this time, there is no option to create a vApp when you are logged in as an OnApp admin user. If you log into an organisation user, you will be able to deploy vApp's on behalf of users there.
This is not possible as it would be created in the same way that a vCloud System Administrator would create a vApp, causing the owner to become 'system'. OnApp does not manage any resources owned by system users.