SPDIF is for enabling digital sound through your HDMI connection.
Since you have a Dell motherboard, you will need to find the SPDIF connection on your motherboard. The location of all connections are easy to find on aftermarket boards, and the manuals they come with, but, since it is a Dell, you probably do not have any such manual showing you the layout of the board.
I went to Dell's support website, selected Optiplex GX280, then manuals, and found the motherboard layout.
It lists no SPDIF connection.
http://support.dell.com/support/edocs/systems/opgx280/en/ug/smtabout.htm#wp1051997
Dude, you got a Dell.
You have two options, forget about running the sound through your HDMI connection (not a problem if you run your sound through a stereo instead of the HDTV).
Or you can get a sound card (most all have an SPDIF connector). You may have to disable your motherboards integrated sound if you buy a sound card, as well as plugging your speakers to the sound card. They're fairly cheap these days.