How to make a <ul> display in a horizontal row

You could also set them to float to the right.

#ul_top_hypers li {
    float: right;
}

This allows them to still be block level, but will appear on the same line.

Set the display property to inline for the list you want this to apply to. There's a good explanation of displaying lists on A List Apart.

List items are normally block elements. Turn them into inline elements via the display property.

In the code you gave, you need to use a context selector to make the display: inline property apply to the list items, instead of the list itself (applying display: inline to the overall list will have no effect):

#ul_top_hypers li {
    display: inline;
}

Here is the working example:

#div_top_hypers {
    background-color:#eeeeee;
    display:inline;      
}
#ul_top_hypers li{
    display: inline;
}

<div id="div_top_hypers">
    <ul id="ul_top_hypers">
        <li>&#8227; <a href="" class="a_top_hypers"> Inbox</a></li>
        <li>&#8227; <a href="" class="a_top_hypers"> Compose</a></li>
        <li>&#8227; <a href="" class="a_top_hypers"> Reports</a></li>
        <li>&#8227; <a href="" class="a_top_hypers"> Preferences</a></li>
        <li>&#8227; <a href="" class="a_top_hypers"> logout</a></li>
    </ul>
</div>