This post asks the same question, but the answer is to use the new template that already makes use of the NugetRestore activity.
I tried to use the new template (TfvcTemplate.12.xaml) on a sample project and couldn't begin to get it to work. I suspect its a rights issue somewhere (can't access the template during the build), but I couldn't solve it. Edit: got a copy of that template and it still doesn't seem to call nuget; same problem - I need to make the NuGetRestore activity work.
In any case I have customizations already in place and only need to add in the new activity. The problem is the activity has a "Solutions" string array property and I don't know how to pass the path to my solution. If I pass {"$(SolutionRoot)\MySolution.sln"}, I see that unexpanded literal in the log: c:\Program Files\Microsoft Team Foundation Server 12.0\Tools\nuget.exe restore "$(SolutionRoot)\MySolution.sln" -NonInteractive
What can I pass as my solution path?