These packages are used for the graphical user interface. You need Tcl/Tk version 8.0 or newer and Tix version 4.1 (or newer ?). In most cases, you have to install them before installing netgen. Tested versions of these packages are available on 3rd party libraries.
The header files are included from the netgen-file "ng4/ngtcltk/incvis.hh". Eventually, you have to replace include<tcl.h> by include <tcl8.0.h> etc.
Tcl/Tk and Tix requires *.tcl - files at runtime. Eventually you have to set the environment-variables TCL_LIBRARY, TK_LIBRARY and TIX_LIBRARY like
setenv TCL_LIBRARY /usr/local/lib/tcl8.2
Togl by Brian Paul and Ben Bederson provides the link between Tcl/Tk and OpenGl rendering. It requires internal tk-header files, which are included for several tk-distributions (like tkInt8.0.h). If you use newer Tcl/Tk versions, please copy the file "tkInt.h" (from the Tk-source directories) to the togl directory (e.g. to tkInt8.2p2.h), and adapt togl.cc (about line nr 120).