[errata=
RMC Spell Law, Page 144]
The Essence is that which is common to all things, living and dead, organic and inorganic. It has been characterized by many names — the Tao, Magic, the Unified Field, etc., and represents a force and order which defines the ways of the world. Changes in the patterns of the Essence dictate the flow of events, and the course of life itself. It is a power beyond the grasp of any being.
[/errata]
Does this mean that undead too have personal essence fields ? If no what do they have instead ?
Personal essence fields? Do you mean for casting spells?
Some undead can cast spells, so the simple answer, taking into account the above quote, would be yes.