Ok if i have a {Lotleth Troll} and use his ability to discard a {Geralf's Messenger} will the the messenger come into play using his undying ability?
No. Undying only triggers when the creature "dies" (is out into the graveyard from the battlefield). Discarding does not count as dying.
RESOLVED
And just for reinforcement...
From the Comprehensive Rules:
702.91. Undying
702.91a Undying is a triggered ability. "Undying" means "When this permanent is put into a graveyard from the battlefield, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it."