it's not enabled by default since in order to make it work you need to do some additional steps, described here:
1. Unpack the zip file into the local/ directory. A new directory will be created called local/mobile.
2. Go to Site administration > Notifications to complete the plugin installation.
3. Go to Site administration > Plugins > Web services >
Mobile, enable "Enable web services for mobile devices" (Remember to
save the changes)
4. Go to Site administration > Plugins > Web services >
External services, edit "Moodle Mobile additional features" and check
the "Enabled" field, then save changes.
5. Go to Site administration > Users >Permissions Define
roles, edit the Authenticated user role and allow the capability