When the dev said subsonic-x.x.deb the "x's" refer to the version number of file. In this case 4.6.
sudo dpkg -i /home/windyridge/Desktop/Downloads/subsonic-4.6.deb
When you literally put in the command:
sudo dpkg -i subsonic-x.x.deb
It could not find the file because no file of that name exits.
As a shortcut, you can substitute the tilde key"~" for the /home/windyridge part so another way to type the command would be:
Hope this helps
sudo dpkg -i ~/Desktop/Downloads/subsonic-4.6.deb
You realize you can also just double click the subsonic-4.6.deb file and it will run the installer for you graphically. It will prompt you for your sudo password before installing anything.