The primary problem is its sheer size (1.4 GB) which tends to fail when being downloaded via KStars Download New Data tool. So here is a quick guide on how to obtain this catalog.
USNO NOMAD requires Tycho-2 catalog to be installed first. It is a relatively smaller download at only 32MB and can be safely installed using the Download New Data tool. Using the keyboard, click Ctrl + N to bring up the dialog, or go to Data → Download.
Navigate to Tycho-2 and click Install.
Wait until Tycho-2 is downloaded and installed. Now download the USNO NOMAD Catalog. Please either use a download manager to download the file, or use wget from the console. To use wget, open a console and type:
Alternatively, you might want to checkout the mirror list first to download the files from a mirror close to you. After downloading the file, extract it and copy USNO-NOMAD-1e8.dat to ~/.local/share/kstars
If you are using console:
tar -xzf USNO-NOMAD-1e8-1.0.tar.gz cp USNO-NOMAD-1e8.dat ~/.local/share/kstars
Now restart KStars, and go to Settings → Configure KStars. You'll see the Star Catalogs density slider, move it up and click Apply. You can control how many stars KStars draw on the screen, the more stars, the more resources it would take to render them, so adjust the slider carefully.