"The problem is with the use of the variable name 'class' in javascript. IE8 doesn't like that.
To fix, rename 'class' to 'class_name':
change lines 62-67 from file:
/administrator/modules/mod_addtomenu/addtomenu/helper.php
var addtomenu_show_start = function( msg, class ) {
", "$( 'addtomenu_msg' )
", ".removeClass( 'success' ).removeClass( 'failure' )
", ".addClass( 'visible' )
", ".addClass( class )
", ".setText( msg );
to:
var addtomenu_show_start = function( msg, class_name ) {
", "$( 'addtomenu_msg' )
", ".removeClass( 'success' ).removeClass( 'failure' )
", ".addClass( 'visible' )
", ".addClass( class_name )
", ".setText( msg );
"