If you are using jsMath as your display mode, you need to update your copy of jsMath. This was added to jsMath in version 3.5 a year and a half ago. The current version is 3.6c. (I think the current version checked into the WeBWorK CVS archive is 3.6b, but that includes the \verb command, so you should be all set with that. Or you could get the most current version from the jsMath web site.)
Davide