Hi,
its been 4 years since i saw this message when implementing the fwsm. so i m not sure, but if i remember right its an "issue the commands in the right order" issue.
Delete all fw statements from the 6500 config
Delete the interface which routes to the fwsm from the 6500
Enter the commands again starting with
firewall multiple-vlan-interfaces
firewall module x vlan-group 1
firewall vlan-group 1 2
interface Vlan 2
ip ....
etc.
Hope it helps, its long ago.