AMP can't discover out-of-the-box IOS APs with SNMP scans because they aren't configured to give enough information over SNMP. There are two things you can do:
1. Use HTTP scans. There is an option on the AMP's APs/Devices ---> Communications page to enable read-write SNMP for APs when AMP manages them. Note that these APs must be managed, have a template and that template must be pushed out to the APs before SNMP scans will work correctly.
2. Log into each AP and change the SNMP Object Identifier to "iso". On the AP's web interface, you can do it on the Services -> SMMP page (on Cisco AP1200 models, other models may vary). On the AP's CLI, log on to the AP via SSH/TELNET, then enter enable mode, then enter config mode, then type these two commands:
snmp-server view iso iso included
snmp-server community iso view public RW
Input "Control-Z" the 'write mem' to save the configuration on the AP.