How To Call JavaScript Function From A Button

I an using a pop out java script and in the notes it says

if later on you want to open / close a specific item you could do it like so:

ExpandingMenu.openItem(3); // toggles item 3 (zero-based indexing)
*/

How to call the function from a button click. I have some button already on my page like so

<li><a href="#">Menu3</a></li>

And from what I have read I should be able to do this:

<li><a href="#" onclick="ExpandingMenu.openItem(3)">Menu3</a></li>

But I don't get any response ..... no error message NOTHING