My VS2010 solution has a test project in it. The unit tests themselves reference the following namespace:
using Microsoft.VisualStudio.TestTools.UnitTesting
The specific answer for me and a Visual Studio 2010 solution was:
The .UnitTestFramework.dll sits in C:\Program Files... folder and you have a reference to it.
After that you can checkin your changes (make sure the Source/Binaries folder is checked in) and build the solution. When build server builds the solution it should get the dll from the source control.