Rulings

Mana produced by Somberwald Sage can be spent on any part of a creature spell's total cost. This includes additional costs (such as kicker) and alternative costs (such as evoke costs).
Mana produced by Somberwald Sage can't be spent on activated abilities, even ones that put a creature card directly onto the battlefield, such as unearth or ninjutsu.
Mana produced by Somberwald Sage can't be spent on noncreature spells that would put creature tokens onto the battlefield.