Teamcity running DotCover

I am trying to get DotCover to run with a build step in our TeamCity builds and not entirely sure how to go about this.

So far I am tryong to to use a command line runner type with the following:-

Command Executable: C:\Program Files (x86)\JetBrains\dotCover\v2.7\Bin\DotCover.exe

Command parameters: cover /TargetExecutable=packages/xunit.runners.1.9.2\tools\xunit.console.clr4.exe /TargetArguments=C:\BuildAgent\work\39cea08ba7c32ed4\Maclean.RosettaNetGateway.Tests\bin\Debug\Maclean.RosettaNetGateway.Tests.dll /Output=.\GregTest.dcvr /TargetWorkingDirectory=.

The problem is that I have is as follows:-

DotCover is only installed on one of the build agents and is working fine when I have then path in the Command Executable as above (i get my dcvr file and it opens and I can see the results which is great)
If I change this path to be %teamcity.tool.dotCover%\DotCover.exe and then run the build step I get a .dcvr file but when I open it, its blank

Can someone shed some light on best practice and why the file is blank? - is it because the %teamcity.tool.dotCover% is an unlicensed path or something?

Thanks
Gregor