The Web Transceiver is a web application for using authorized AllStarLink nodes. Unfortunately, it is ancient and insecure Java software that is no longer supported. It may not work well or at all but it's the only Web AllStar client around so give it a try. The attached PDF is the sum knowledge regarding the installation of the Web Transceiver. If these don't work you may be out of luck. Viable alternatives are IAXRPT or Zoiper, provided the node owner/admin has given you access.
These setup instructions detail install and setting up Java to work with the Web Transceiver. Both Windows and Mac instructions included.