You will have to extend the keyboard navigation functionality on combotree component. Here is the simple implementation of doing so.
(function(){
$.extend($.fn.combotree.methods,{
nav:function(jq,dir){
return jq.each(function(){
var opts = $(this).combotree('options');
var t = $(this).combotree('tree');
var nodes = t.tree('getChildren');
if (!nodes.length){return}
var node = t.tree('getSelected');
if (!node){
t.tree('select', dir>0 ? nodes[0].target : nodes[nodes.length-1].target);
} else {
var index = 0;
for(var i=0; i<nodes.length; i++){
if (nodes[i].target == node.target){
index = i;
break;
}
}
if (dir>0){
while (index < nodes.length-1){
index++;
if ($(nodes[index].target).is(':visible')){break}
}
} else {
while (index > 0){
index--;
if ($(nodes[index].target).is(':visible')){break}
}
}
t.tree('select',nodes[index].target);
}
if (opts.selectOnNavigation){
var node = t.tree('getSelected');
$(node.target).trigger('click');
$(this).combotree('showPanel');
}
});
}
});
$.extend($.fn.combotree.defaults.keyHandler,{
up:function(){
$(this).combotree('nav',-1);
},
down:function(){
$(this).combotree('nav',1);
},
enter:function(){
var t = $(this).combotree('tree');
var node = t.tree('getSelected');
if (node){
$(node.target).trigger('click');
}
$(this).combotree('hidePanel');
}
});
})(jQuery);