my fontButton's |menu|()'s addItemWithTitle_action_keyEquivalent_("", "changeTheFont:", "")
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# I'd already tried this, but it doesn't work.
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my fontButton's |menu|()'s last item's setAttributedTitle_(eachAttributedFontString as text)
So you're calling the method on *something*, and you know it should be a menu item. But "last item" is hardly going to work, because that's for AppleScript lists, not NSMenus. So the question becomes: what method of NSMenu will return you its last menu item? And there isn't one, directly, but there are several that return menu items. Your choice becomes: (a) use itemAtIndex: and keep count so you know the index, (b) use itemAtIndex: and call numberOfItems every time, which is probably going to slow things down, or (c) do something creative like make your menu item with a placeholder title, and use itemWithTitle:. The latter will work, because you're going to effectively overwrite the title each time.
An alternative is to create the menu item first, using alloc and initWithTitle:action:keyEquivalent:, set its attributed title, and add it to the menu using NSMenu's addItem: method.