ok, here is the line from my skin

<dnn:HOUSEMENU runat="server" id="dnnHOUSEMENU" Scope="-1" ShowAdmin="False" ShowHome="False" IsRecursive="False" Orientation="H" Mode="S" CssElementId="HouseMenuNav" /></div>

And here is the styling inside my skin.css. I think the piece your missing is CssElementId is not the name of the CSS file its the name of the object/class/selector in any CSS file associated with the skin.

Here is the CSS in my skin.css which controls the formating of my HouseMenu

/* ===========================================
Horizontal Menu
==============================================*/
#HouseMenuNav
{
    /*menu wrapper div */
    margin-left: 40px;
    font: bold 120% Arial, Tahoma, Verdana sans-serif;
    padding: 0 0 20px 0;
}
#HouseMenuNav ul
{
    border: 0;
    margin: 0;
    list-style: none;
    float: left;
/*Background-color can be added here */
   
}
#HouseMenuNav li
{
    float:left;
    list-style: none;
    padding: 0 0 0 0;
    background-image: url(buttons/MaroonWhiteGrdnt_default.gif);
    background-repeat: repeat-x;
    background-color: #990000;   
}
#HouseMenuNav li a:link, #HouseMenuNav li a:visited
{
    display: block;
    color: #FFF;
    text-decoration: none;
    padding: 0 5px 0 5px;
    border-right: 1px solid #990000;
}

#HouseMenuNav li a:hover
{
    background-image: url(buttons/MaroonWhiteGrdnt_hover.gif);
    background-repeat: repeat-x;
    background-color: #990000;
    color: #FFF;
}
#HouseMenuNavCurrentLink
{
    background-image: url(buttons/MaroonWhiteGrdnt_selected.gif);
    background-repeat: repeat-x;
    background-color: #e57b7b;
    cursor: default;
}
   

As you can see my skin.css is where it pulls from that then has a Selector #HouseMenuNav  which is what I have in my CssElementId.

Incase your  curious #HouseMenuNavCurrentLink is special Selector which controls the display of the selected tabs menu object. Note it doesn't format Home tab, though you have to use #HouseMenuNavItemHome. But lets first get your menu working then we will have some extra fun. :)

You can link house menus together a commonly asked question by creating multiple instances of the menu and setting the scope to match a the tab Id of the previous menu you want tied to it. For example I have a main menu across the top of my page which has 6 main categories. On the left side of my page I display the child pages of the main category when the user selects a category it changes the left menu to only display the child parents. All you do is visit the page id of the main category and grab the tab id from the url. From there you just set the scope of the categories skin to be the tab id. Then set all the child pages to share the same skin as the main category.

You will find Housemenu is a bit "limiting" at first but its so flexible that you can really build any menu system you want with it, just have to think outside the box. Heck one site I have has 6 HouseMenus on it 4 of them tied together and with faster servers my users thinks they are clicking on special page controls when really its just swaping skins around to display different modules etc. I wish I could show you but its a private companies site. If you ever want to do something and hit a road block on how to do it just let me know.