The USGS WMS for scanned topo maps 1:24,000 seems to be gone for good. It's down for a couple of weeks now, though it's still mentioned in the USGS online service catalogue, but the link to the address has been removed. I did a bit of research now and found out that this service was hosted by ESRI's ArcGIS Online portal. Digging further I discovered a Map Tile service providing the same data. I decoded it (simple enough, it follows the Open Street Map scheme) and created a set of TransDEM map tile parameters for it.
Copy the following code section into a text editor, save as an xml file and import into TransDEM with the Map Tile Client / Settings.Code:
<?xml version="1.0" encoding="iso-8859-1" ?>
<TileServer version="1.0" >
<Provider name="_10 USA Topo" >
<URL addr="http://services.arcgisonline.com" prefix="ArcGIS/rest/services/USA_Topo_Maps/MapServer/tile" standalone="1" type="1" >
<Matrix prefixClm="/" prefixRow="/" prefixZoom="/" minZoom="1" maxZoom="18" order="3" />
<Mercator origLat="90" origLon="-180" signLat="-1" signLon="1" projection="4" />
</URL>
</Provider>
</TileServer>