The installation process of Viewfinder is a little bit — but only very little tiny small bit — more complicated than the one of other plugins. First of all, download the JAR file from the official download page. Then place the file in your Bukkit plugins/ folder. If you have rented a Minecraft server on a remote machine and you can't access the filesystem directly, use FTP or SSH-Copy/SCP to upload the file.

If you start the server for the first time with Viewfinder, it will automatically download another plugin called QuarterBukkit. This is necessary for Viewfinder to run. After the download process is finished, you're ready to go!