VSTA doesn't support integrated source control, so you'll probably need to
use Visual Studio 2005 with VSTO if you want to use SourceSafe for version
control. There's a section in the InfoPath 2007 programming documentation
that provides instructions on how to convert a VSTA project to use Visual
Studio.

If VS2005 is not an option, then you are probably stuck with using the
SourceSafe client program to check-in/check-out files. You must check out the
.XSN file as well as all files in the folder/subfolders containing the VSTA
solution for the form template.