If you look in the group/user setup page of ACS you'll see a section "IP Assignment"
For each group of users, just select the "Assigned from AAA Client pool" and enter the pool name.
When users authenticate, ACS will send the pool name back to the device.
This is protocol independant and to some extent RADIUS vendor independant too in that it knows about IETF, Cisco and Ascend assignment methods.
