1. Login as admin and go to Settings > Site administration > Plugins > Web services > External services.
2. Tick Enable mobile web services and Save.
3. Check the following:

• Site administration > Advanced features : web services should be enabled
• Site administration > Plugins > Web services > External services : mobile external service should be listed and enabled (not greyed out)
• Site administration > Plugins > Web services > Manage protocols : xml-rpc protocol is enabled
• Site administration > Users > Permissions > Define roles > 'webservice/xmlrpc:use' is allowed for the authenticated user role

See also http://docs.moodle.org/20/en/Enable_mobile_web_services