My client paid the upgraded fee for CiviCRM Spark Hosting with CiviMail and Mosaico. CiviMail is working, but Mosaico isn't under Mailings as it is in other CiviCRM instances I work with. I also can't find anywhere to enable it.
Unfortunately, it appears Spark users can't install additional or upgrade Extensions, yet. However, if the site was upgraded to add Mosaico, that should appear as an Enabled extension at Administer -> System Settings -> Extensions and at Mailings -> Mosaico Templates. If it does not, please contact the Spark folks at chat.civicrm.org in the ~spark channel. You may also want to run Administer -> System Settings -> Cleanup Caches.