Nagaraju,
vCenter need push one vpxa agent package to ESXi server and regarding this issue, I have two comments.
1. The vCenter server's version should be higher than ESXi server's verison.
Here you used vCenter 4.x, which could manage ESXi 3.x/3.5/4.x. you need install one vCenter 5.x to manage your ESXi 5.x.
2. communication between ESXi/vCenter Server.
Both server could pingable each other and nslookup utility should parse two server's ip/hostname before you add ESXi server to vCenter.
By the way, connecting to ESXi server directly using vSphere client, works?
Best Regards,
changhai