Although you haven't explained what you have done so far to get that error I guess you haven't imported the appropriate libraries. Make sure you import all the libraries that come in the HTMLUnit distribution.
You can take a look at how to do this in Eclipse in the following question, maybe it can help: How to setup HtmlUnit in an Eclipse project?
By the way, there is no need to switch to any old version. Just import the libraries and you're done.