Which mobile OS you are trying it on? I just tried it android phone (4.2+) and i am able to login.
For Android :
https://play.google.com/store/apps/details?id=com.jivesoftware.mobile.nativeapp.android
iOS: https://itunes.apple.com/us/app/jive-mobile/id492118906
(Not compatible with iOS 8)
Regards,
JK