Not sure what you mean by "pop-up/drop-down/flip-out," but I've used HouseMenu on a couple of (single purpose/use) skins.

If you've already got it working (eg - outputting the unordered lists), you just need to apply a decent stylesheet to it.  Check out the original A List Apart article or the updated Son of Suckerfish article at HTMLDog for that.

I've had some problems with getting HouseMenu to send the javascript needed for IE6/7, but that can be fixed by using the following in your skinpath:

<!--[if LTE IE 7]>
<link rel="stylesheet" type="text/css" href="<%= skinpath %>IE.css" />
<script type="text/javascript" src="<%= skinpath %>suckerfish.js"></script>
<![endif]-->

Where the IE.css file is any IE specific CSS you need for the skin and the suckerfish.js is the javascript code found at either of the articles I listed above.  The code I listed above uses conditional comments to keep your skin (and attached CSS files) compliant with whatever standards you choose to adhere to.