For each site that will be using the Mobile Device functionality, you need to turn on the feature in the Site Details. Once you turn the feature on, you will be able to proceed with configuring the different components that support this feature.
From the System menu, go to the Sites manager.
Select a site by clicking on the Site Name link.
In the Site Details section, check the Enable Mobile Device Support checkbox.
Click Save.
Proceed with further mobile device configuration using the procedures noted below.