I have seen the html-source of your page here http://www.oscommerce.com/forums/index.php?showtopic=200249#
Your shop is placed in the subfolders, isn't it?
href:"http://localhost/ms2fr/catalog/index.php?cPath=1_36_37
YahooTreeMenu does not work in subfolder jet. Sorry.
Also maybe you had changed yahootreemenu.php.
Look here, please.
var request = "yahootreemenu.php?cPath=" + node.data.id;
MUST be
var request = "categories.php?cPath=" + node.data.id;
Because of that yahootreemenu dosn't work in your shop.