The plug in message most likely refers to Java, and it sounds like you may need to update to the current version of Java for the image uploader.
You can check this at http://java.com/en/download/index.jsp
- where there is a link: "Do I have Java?" under the download button, where you will be able to see which version you are running and update it if needed. Also note that the download and install are separate steps.
However, the HTML uploader does not require Java, so if you are having difficulty with that or issues with uploading using the Auctiva uploader, please let us know what is occurring for you in a support case that you can file from under the Help tab of the site and we'll be happy to look into it.