This is because the IP address of the system returned by the SNMP Agent is wrong.
Stop Applications Manager
Go to <Applications Manager Home>/working/conf> directory & take a backup of the seed.conf file
Edit the seed.file present there using WordPad or vi editor, change <ENABLE_SNMP_DISCOVERY="true"> entry to <ENABLE_SNMP_DISCOVERY="false">
Restart the Applications Manager and add the monitor again.