Without html I can't tell you which is the appropriate element to add the z-index but you should do this:
If the menu and the buttons are siblings then simply add position:relative to the menu container and then add a high z-index. Next add position:relative to the button container and set a lower z-index than the menu.
If the elements aren't siblings then use the same approach on the parents of those elements that are siblings.
Note that only positioned elements can have z-index applied so if the element is static then you need to add position:relative as well.