The latest version of Paper Machines is 0.4.11. To install in Firefox, click this link. To install in Zotero Standalone, right-click (control-click) the link above and save the XPI file in your Downloads folder. Then, go to the Tools menu in Zotero -> Add-Ons. Select the gear icon at top right, then “Install Add-On From File.” Navigate to your Downloads folder (or wherever you have saved the XPI file) and open it.