Nunit tests fails when running continuous integration build on team foundation service

Nunit tests fails when running continuous integration build on team
foundation service

I have followed a couple of guides on how to get nunit to work on TFS (the
cloud version)
http://www.mytechfinds.com/articles/software-testing/6-test-automation/72-running-nunit-tests-from-team-foundation-server-2012-continuous-integration-build
http://walkingthestack.blogspot.sg/2013/04/using-nunit-for-your-tests-in-team.html
However, after setting up everything, I'm still getting this error message:
Exception Message: The path '$/Plan.Ess.sln' could not be converted to a
local path. Make sure this path is relative to the 'src' folder on the
build machine or specify a full server path. (type ArgumentException)
Exception Stack Trace: at
Microsoft.TeamFoundation.Build.Activities.Core.LocalPathProvider.GetLocalPath(String
incomingPath)
at System.Activities.CodeActivity`1.InternalExecute(ActivityInstance
instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
at
System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor
executor, BookmarkManager bookmarkManager, Location resultLocation)
TF270003: Failed to copy. Ensure the source directory C:\a\bin exists and
that you have the appropriate permissions.
1) $/Plan.Ess.sln is what I keyed in my Build Definitions > Edit Build
Definition > Process > 1. Solution to build
2) I'm not sure why it's refering to C:\a\bin
I'm not sure if I have "Make sure that installed test discoverers &
executors, platform & framework settings are appropiate and try again"
(Solution from the 2nd link) But I have already pointed the build
controller to the folder containing the nunit test adapters & dlls