To use Tix with other TCL extension packages, you have to call the function Tix_Init() in your Tcl_AppInit() function. Here is an example:
int Tcl_AppInit(interp)
    Tcl_Interp *interp;
{
    Tk_Window main;
    main = Tk_MainWindow(interp);
    if (Tcl_Init(interp) == TCL_ERROR) {
	return TCL_ERROR;
    }
    if (Tk_Init(interp) == TCL_ERROR) {
	return TCL_ERROR;
    }
    if (Tix_Init(interp) == TCL_ERROR) {
	return TCL_ERROR;
    }
    /*
     * Call the init procedures for included packages. 
     * Each call should look like this:
     *
     * if (Mod_Init(interp) == TCL_ERROR) {
     *     return TCL_ERROR;
     * }
     *
     * where "Mod" is the name of the module.
     */
}