button dropdown click an item

I read this article but it did not help after all. I want to click an item from the button dropdown with this HTML markup:

<div class="input-group-btn" id='mainCatDropdown'>
                    <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-expanded="false" >
                        Kategorie hinzufügen
                        &nbsp;<span class="caret"></span>
                    </button>
                    <ul class="dropdown-menu" role="menu">
                                                        <li id='mainLi1'><a href='#' onclick='addMainCategory(1); retu false;' id='mainCat1' role='menuitem'>Entwicklung</a></li>
                                                        <li id='mainLi2'><a href='#' onclick='addMainCategory(2); retu false;' id='mainCat2' role='menuitem'>Grafik, Content, Medien</a></li>
                                                        <li id='mainLi3'><a href='#' onclick='addMainCategory(3); retu false;' id='mainCat3' role='menuitem'>IT-Infrastruktur</a></li>
                                                        <li id='mainLi4'><a href='#' onclick='addMainCategory(4); retu false;' id='mainCat4' role='menuitem'>Beratung &amp; Management</a></li>
                                                        <li id='mainLi5'><a href='#' onclick='addMainCategory(5); retu false;' id='mainCat5' role='menuitem'>SAP</a></li>
                                                        <li id='mainLi6'><a href='#' onclick='addMainCategory(6); retu false;' id='mainCat6' role='menuitem'>Ingenieurwesen</a></li>
                                                </ul></div>

All I need is to click a specific item from the button dropdown. Any ideas?