The browser interface uses a browser. There isn't anything to download. The link references the 5250 portlet, which you would put on a page served by your WebSphere Application Server. The WebSphere Portal InfoCenter will help you understand how to set the authentication up. Especially see Securing your portal > Security concepts > Authentication. Here is the home page for IBM i Access for Web. In particuklar, check out the documentation link at the bottom.
If you are V5R4, be advised that support is about to elapse.