I disabled the firewall on the client and the server and was able to ping between the two, but WDM would not detect the client.
Install the updated agent 6.1.25 check your DHCP options within Configuration Manager (Perferences) Check subnets within the Network tap make sure the TC your looking for is listed on a subnet your descovering, DNS needs to resolve. What happens when you run a manualy descovery?
Dell will deny it, but WDM is horribly broken, even in 5.0. I encourage you and others to keep after them in these forums and elsewhere, until they start to seriously address the issue.