The "Unable to add the system. Kindly check whether it is reachable" message is displayed because you are not able to reach the system in any one of the following ways .
1)ICMP ping. Execute the below command in the command prompt and check if ICMP ping is working.
Check whether SNMP Agent is running in the remote system you are trying to monitor.
3)Native ping. This is disabled by default.
Solution: Enable any one of the above 3, so that you would be able to reach the system.
To enable native ping, Edit <Appmanager/conf/AvailabilityTests.conf> and Change <am.enablenativeping=false> to <am.enablenativeping=true> [if you change the option as true then start Applications manager as root in linux and admin user in Windows ]