This is currently being rolled out and as per the release notes it may take a week to enable this on all customers instances.
"Mobile Friendly Landing Pages
You can now build mobile views for landing pages from within the landing page editor. Deliver your message effectively regardless of device and increase engagement by tailoring your content for easy consumption on-the-go. This feature will rollout gradually throughout the week following the release. "
If you would like this enabled prior to this, please Open a case with the Support team to turn this on for you here: https://community.marketo.com/MarketoSupportHome