You can't ping the machine from the ePO server till its managed.
when you are adding the machine to the ePO server, you are simply creating entry in Leafnode and it doesnt have any info about the client machines.
Is agent deployed to the client machines? If not then please refer to KB#: KB56386 for the environmental requirements for deploying the agents.
In addition to this, make sure that apache.exe is released from the firewall (from Win firewall) for ping function of ePO to work.