For JBidwatcher to log anything about its operation, you have to have Logging enabled in the General tab of JBidwatcher's Configuration Manager. If it's logging, then you can find the error logs by looking in JBidwatcher's home directory for files named error.##.log (where ## is a number).
On Windows it's usually:
C:\Documents and Settings\{username}\.jbidwatcher
On the Mac it's at:
/Users/{username}/Library/Preferences/JBidwatcher
On Linux, it's just:
~/.jbidwatcher
You can view it with more, or any text editor in Linux. On Windows, Notepad or Write will do the job. On Mac OS X, if you double-click the files it will open Console.app, which will work very well.