I'm having sign-in problems

If you're having a login failure, you'll usually see your eBay username in strikeout up in the upper right of JBidwatcher, or the message 'Not logged in...'. You may also see a red icon, which indicates a login failure. If you move your mouse over that area, it'll tell you what caused it to think there's a login failure. That information is usually very helpful in figuring out what's wrong.

If you're having sign-in problems, the most likely issue is eBay putting up a captcha, in which case you need to actually stop using JBidwatcher for a day or so, log in normally (through your browser) and let eBay's security system relax about your account.

If you're running Windows, it may be a different kind of security failure. First, upgrade your version of Java by going here:
http://java.sun.com/getjava and install that. It may help, especially if you're seeing something about SSL or certificates when hovering over the login failure.