for a menu item do something like:
menu "Image/ColorBars" ColorBars
for a new icon in the toolbar set which toolbar you want to use first:
set bar
example:
set bar top
then define the icon itself:
toolbar
example:
toolbar "ColorBars" ColorBars.xpm ColorBars
All of this can go into your menu.tcl or any other tcl file that is in your NUKE_PATH
To see how these commands are used to build Nuke's native UI have a look at the menu.tcl and the toolbar.tcl in Nuke's plugin directory (the one it installs with).
hope this helps.
frank