AddIn exception: The solution's source code database may not have been opened.

I have installed on my computer the Google Test Add-in:

http://googletestaddin.codeplex.com/

It "works on my machine"... 

On a different computer though, with the exact same configuration, so it seems to me, its buttons are grayed out. Running it through the Debug, I got

in the QueryStatus,  on the second "if" line

if ( neededText == vsCommandStatusTextWanted.vsCommandStatusTextWantedNone )
{
  if (	applicationObject == null || applicationObject.ActiveDocument == null || 
applicationObject.ActiveDocument.ProjectItem == null ||
applicationObject.ActiveDocument.ProjectItem.ContainingProject == null ||
( applicationObject.ActiveDocument.ProjectItem.ContainingProject.CodeModel.Language != CodeModelLanguageConstants.vsCMLanguageVC &&
applicationObject.ActiveDocument.ProjectItem.ContainingProject.CodeModel.Language != CodeModelLanguageConstants.vsCMLanguageMC ) )
  {
    statusOption = vsCommandStatus.vsCommandStatusUnsupported;
    return;
  }
//...
}

I don't know how to troubleshoot this problem... I tested this to see if it may be that perhaps some sdk was not installed - by running it in a virtual environment that didn't even have room for SP1, but I had no problem with it.

What does this error mean, and how can I fix it ?

I am running Visual Studio 2010 (SP1).

Thank you.