The answer is in the NCSA X-Mosaic FAQ under "Something Doesn't Work 
Right." The FAQ is at the URL:

http://www.ncsa.uiuc.edu/SDG/Software/XMosaic/faq-broken.html

To summarize its answer, it says to:

1. Put a copy of XKeysymDB in /usr/lib/X11
2. Override the translations for the Motif Text and TextField fields to
provide backspace functionality by putting the following lines in your
".Xdefaults" file in your home directory:

*XmText.translations: #override\n\
	<Key>osfDelete: delete-previous-character()
*XmTextField.translations: #override\n\
	<Key>osfDelete: delete-previous-character()

Elsewhere on the same page, the FAQ advises you to set the environment
variable 

XKEYSYMDB=/usr/lib/X11/XKeysymDB

and export it (if you're using bash) to keep Mosaic from dumping loads 
of keysym translation table errors when starting Mosaic.

Thad Salmon
Lawrence Livermore National Laboratory
salmon2@llnl.gov