This problem can be due to either one of the below reasons.
(a) Microsoft .Net Framework not installed in your system.
(b) A lower version of Microsoft .Net Framework installed in your system. The tool requires Microsoft .NET Framework 3.5 and above for running. The bin directory of the tool is provided with a utility ‘DotNetUtilities.exe’. Run this utility from the command prompt to find the .Net version installed in your system.
You can download and install Microsoft .Net framework 3.5 from here.
XenServer Tools must be installed in all the VM's you are interested in monitoring, then only you will be able to get the metrics(otherwise XenServer won't collect the performance metrics) for the VMs.