I managed to get data out of them based on this write up:
First, the current URL is:
http://viewer.nationalmap.gov/viewer/Go there and find your area of interest. Navigation controls are similar to Google Maps. Ideally it should not be a really huge area.
Now click Download Data in the upper-right. You should then get a little pop-up window with some options for choosing what to download. I like "Draw and download by bounding box', but any of them are potentially useful, and in particular if you have zoomed all the way in to only show the *exact* area you want, then "Download by current map extent" may be good.
Once you select your download area, another window should pop up showing download data options. Check Elevation (second from bottom), and any other data you want. Then press Next.
You will then get a list of available data. There are usually a lot of similar options. The two major differentiators are 1: resolution (under the Resolution column), which is generally measured in arc-seconds, the smaller the number/fraction, the higher the resolution and bigger the files for equivalent area, and 2: format (format column), where you'll find there are lots of the exact same data (same resolution) just in different formats such as ArcGrid and IMG. The good news is Terragen 3 supports almost every format they offer, particularly in terms of heightfield/elevation data, so it doesn't really matter what you choose. Just find the data you want at the resolution you need (usually the highest available, unless file/download size is a concern for you), check it in the box to the left of that item, then click Next at the bottom when you've selected all you want (downloading multiple resolutions is not necessarily bad, but no need to download multiple formats of the same data).
Once selected your data option(s) should show up in the Cart to the left. Then press Checkout. You'll need to enter an email address, don't worry they won't spam you.
You should then get an email with download links.
