Several simple examples have been posted to comp.lang.tcl.  Eric Bleeker
ericbl@paramount.nikhefk.nikhef.nl was gracious enough to package his up
and place it at
ftp://ftp.neosoft.com/pub/tcl/alcatel/code/tkHelloWorld*. This program
assumes that you have installed the Tcl and Tk libraries, header files,
etc.

Alternatively, you can look into creating a dynamically loadable library
of your C code for Tcl/Tk.  There are some basic examples for all
platforms at
ftp://ftp.scriptics.com/pub/tcl/misc/example.{zip,tar.gz,tar.Z}.