The ability to select the Secret Ballot option is available in the Ballot Results section of the Add a Ballot screen, as shown below:
If you select 'Configurable', you'll be presented with several options to display ballot information, both while the ballot is in process and after it closes. (Results availability, sharing and style are all configurable options.)
Selecting 'Administrators Only' will not offer you configuration options, however.
Reference: Secret Ballots (KB article)