Where is the source code if I have downloaded a binary distribution?

The Tcl code is never compiled.
You can find it in the http://splweb.bwh.harvard.edu/cgi-bin/develop/viewcvs.cgi/slicer2/Base/tcl/tcl-main/, http://splweb.bwh.harvard.edu/cgi-bin/develop/viewcvs.cgi/slicer2/Base/tcl/tcl-modules/, and http://splweb.bwh.harvard.edu/cgi-bin/develop/viewcvs.cgi/slicer2/Base/tcl/tcl-shared directories.
The C++ code is in slicer2/Base/cxx in the CVS repository, but it is not distributed with the binary packages.