The category menu uses the asp treeview control I just googled your question and it doesnt look likes its possible to display the treeveiw horizontally
http://forums.asp.net/t/952732.aspx
Alternatively you could add an html module to the page and create hyperlinks passing the category id in the querystring. See the following how to forum post:
http://www.smith-consulting.com/forums/forumid/13/threadid/3577/scope/posts.aspx