[css] How can I make content appear beneath a fixed DIV element?

Wrap the menu contents with another div:

<div id="floatingMenu">
    <div>
        <a href="http://www.google.com">Test 1</a>
        <a href="http://www.google.com">Test 2</a>
        <a href="http://www.google.com">Test 3</a>
    </div>
</div>

And the CSS:

#floatingMenu {
    clear: both;
    position: fixed;
    width: 100%;
    height: 30px;
    background-color: #78AB46;
    top: 5px;
}

#floatingMenu > div {
    margin: auto;
    text-align: center;
}

And about your page below the menu, you can give it a padding-top as well:

#content {
    padding-top: 35px; /* top 5px plus height 30px */
}