I visited the admin page and when I tried to click on Add Stock or Add Model on the sub-menu for Add Items, I noticed that they "disappeared". I then used the Web Developers Toolbar in Firefox to edit the CSS to see what was going on, and I see that you have this selector defined:
/* Sets the style for a link that is being activated/clicked. */
in this file:
When I removed the float: left; from that selector, then the links seemed to work fine.