Make sure your security settings are not set too high. IE, allows you to edit your security settings from the Security tab in the IE Properties dialog box. Netscape does not have a similar preference.
If you installed the 4.5.1 Plugin over the 4.5 Plugin, you may also experience Java errors.
Try uninstalling the Plugin using the Add/Remove control panel, and then reinstall. If that doesn't help, move your bookmarks and address book to a temporary location, uninstall and reinstall your web browser, and then reinstall the Plugin.