I finally found the link to download the MPEG-4 and HD plug-ins for VS9. I just tried the MPEG-4 and it is still functional.
You must have your version, a full version, not an SE or VE version registered wtih Ulead.
1 Make sure your POP-UP Blockers are disabled. Then enter the serial number to VS9 where it asks for it, press submit. A pop-up window will open where you should be able to download it.
2. To install, you must be using a Full version, not an SE or VE. SE and VE are free versions, that come either bundled with hardware or are found in magazines.
http://www.ulead.com/events/vs9_hdv/runme.htm
Since you state that it will not install, could you post what version you have? Click on the ? and select About. The version number should be shown there. If it as a SE or VE as a part of the version, then it does not qualify for the MPEG-4 or HD plug-in.