Just extract the contents of the plugins folder in the .xpi archive (http://www.go-mono.com/moonlight/download.aspx to /usr/lib/mozilla/plugins/. I just tried it and it works.
On another installation I had to put it in /home/user/.mozilla/plugins/moonlight/silverlight-media-pack-linux-x64-21-1.so
I had to install the codecs as root via sudo firefox
Alternative
Most of the time gecko-mediaplayer will be a good alternative if you don't want to use closed, propriety software.
sudo pacman -S gecko-mediaplayer
On another installation I had to put it in /home/user/.mozilla/plugins/moonlight/silverlight-media-pack-linux-x64-21-1.so
I had to install the codecs as root via sudo firefox
Alternative
Most of the time gecko-mediaplayer will be a good alternative if you don't want to use closed, propriety software.
sudo pacman -S gecko-mediaplayer
