Some recent Compositor release announcements included an incorrect download link for the Windows signing certificate. The links have now been corrected.
If you had trouble downloading the certificate while installing Compositor for Windows, please use the corrected link: Download Compositor.cer.
After downloading, double-click the certificate, choose Install Certificate, select Local Machine, and place it in the Trusted People certificate store. Then retry the Compositor installer.
Sorry for the inconvenience, and thank you for your patience!