By any chance do you have multiple IP addresses assigned to the NIC on your Windows PC? This is currently a know issue in the current shipping version of MAX (note this should be fixed in our 14.0 release). If so, you could try to remove the secondary NIC from your PC.
The other thing that can be problematic is firewalls on your PC. You may want to open up your firewall settings to make sure MAX isn't being blocked. You could also testing this by temporarily disabling the Windows firewall and then relaunching MAX to see if it helps you discover the target.
Some of the firewall settings information is covered in this document.
http://www.ni.com/gettingstarted/setuphardware/compactrio/troubleshootmax.htm
