http://purl.org/tcl/home/man/tcl8.4/TkCmd/messageBox.htm
Note: Unlike most Tk widgets, this widget's name (messageBox) is not the same as its creation command (tk_messageBox).
incr Widgets also has a messageBox - once their DNS problems are resolved, a URL to their doc will be inserted in place of this part of the msg.
Tk syntax help - Arts and crafts of Tcl-Tk programming - Category Command