I read the readme, and it said that I could get the Jabo plugin from PJ64. But where is it located in PJ64? It's not in the plugin file, and that's the only place I can think of looking for it. Is there any way I can download the plugin directly?
No, the Pj64 authors have specifically asked emulation related news sites not to distrubute the plugins on their own, they were created as a part / and will remain a part of Project64. Head over to www.pj64.net and download all 4 versions, 1.0-1.4. Extract them separate directories and open up their plugins folders. Copy all files in the folder to the folder you want, in this case 1964's plugin folder.