par Juan Leyva.
Hi,
have you enabled mobile services in your installation?
http://docs.moodle.org/24/en/Mobile_app#Moodle_sites_must_be_enabled_for_the_app_to_access_them
have you read the FAQ? http://docs.moodle.org/24/en/Mobile_Moodle_FAQ
Most of the problems are due to you are using authentication methods like LDAP SSO, Shibbolleth etc..