In Chrome on OSX (do not have a PC to test on at the moment), clicking on the "[+]" that the .menuTree:before class provides does not collapse/expand the tree. Clicking on the text in the link does.
Safari and Firefox display the expected behavior of expanding/collapsing on clicking the "[+]".
You can see this behavior in the example page on http://menutree.pixelhandler.com/jQuery-MenuTree-Plugin/example/
I see the issue, the [+] behavior like a button is meant to be an example not necessarily production usage. A while back I implemented this plugin here : http://www1.downloadoffice2010.microsoft.com/usa/faq.aspx?culture=en-US this seems to still work in Chrome when clicking the arrows next to each faq item. Perhaps take a peek at the source there as reference. best regards, -Bill