It sounds like you don't have the mysql database driver in your classpath.  This is a separate download from the web.

From: users-bounces@xwiki.org [mailto:users-bounces@xwiki.org] On Behalf Of Campbell, Timothy D
Sent: Friday, October 26, 2007 1:39 PM
To: XWiki Users
Subject: Exception while reading document XWiki.XWikiPreferences

 
After installing both my servlet container and Mysql database, I am still not able to log into xwiki. I edited the necessary files and am positive that Tomcat 5.5 is configured correctly. I can get the apache page but when I click xwiki home I get the error listed below. One strange thing is when I comment the mysql database back, it allows me to view the xwiki but when I uncomment like the directions say, it does not work. What could be the problem?
Thanks,
Tim
 
javax.servlet.ServletException: Error number 3 in 0: Could not initialize main XWiki context
Wrapped Exception: Error number 3202 in 3: Exception while reading document XWiki.XWikiPreferences
Wrapped Exception: Could not create a DBCP pool
	org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:535)
	org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:433)
	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
	com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:117)

root cause 

com.xpn.xwiki.XWikiException: Error number 3 in 0: Could not initialize main XWiki context
Wrapped Exception: Error number 3202 in 3: Exception while reading document XWiki.XWikiPreferences
Wrapped Exception: Could not create a DBCP pool
	com.xpn.xwiki.XWiki.getMainXWiki(XWiki.java:247)
	com.xpn.xwiki.XWiki.getXWiki(XWiki.java:397)
	com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:94)
	org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
	com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:117)