You need the ASA to pass traffic through the IPS as explained in http://supportforums.cisco.com/docs/DOC-5668
Note that since this is a different module you will need something like
access-list ips-acl extended permi ip any any
class-map ips-class
match access-list ips-acl
policy-map global_policy
class ips-class
ips fail-open
service-policy global_policy global
Then you can start configuring the ips following its config guide to fire on signatures etc.
I hope it helps.
PK