To use DHCP to assign addresses for VPN clients, you must first configure a DHCP server and the range of IP addresses that the DHCP server can use. Then you define the DHCP server on a tunnel group basis. Optionally, you can also define a DHCP network scope in the group policy associated with the tunnel group or username. This is either an IP network number or IP Address that identifies to the DHCP server which pool of IP addresses to use.
Try thsi link for configuration
http://cisco.com/en/US/products/ps6120/products_configuration_guide_chapter09186a0080450bf1.html#wp999516