Netscape 7.1 has a list of allowed ActiveX controls. In order to host UMedia ActiveX control, it's ClassID needs to be added to the file "activex.js", found in Netscape's installation folder, typically "C:\Program Files\Netscape\Netscape\defaults\pref".
Please either edit your local "activex.js" manually, or replace it with the ready one that this package includes. Restart Netscape.