Hello David -
If you want this information with respect to Visual Studio - then VSTO 3.0 is part of the Visual Studio 2008 suite. You will have to choose the appropriate options during install.
And note that you have to download the VSTO 3.0 for Office 2007 separately, if you do not have Visual studio. The download link is
here.
However, As per my knowledge - VSTO 2005 SE is not part of Visual Studio setup.
BR