In the Glossary it states
.Ascension Chakra - This is the Sixteenth Chakra which is the beginning stage of realizing fifth dimensional reality. The Sixteenth Chakra is given this name because this is the beginning stage of what is often known as the ascension process.
The book explains that you can access this chakra by needling the head in a specific order.
Cyndi Dale places it on the spine in the dorsal vertebrae (point 16).