Review the CECM web pages entitled Mixing Console Basics and Mixing Console Troubleshooting (which includes Effects Sends).
NOTE: Insert your CD into the drive built into the computer. (Press and hold the eject key, in the upper right corner of the Mac keyboard, to open and close the CD tray). The iTunes program should appear, with the CD tracks listed. If it doesn't, just double-click one of the "files" on the CD.
The CD sound comes into the mixer as line inputs, on the channels labeled "MOTU Main Out."
(The setting that determines where the CD plays is in the Sound panel of System Preferences. Click the System Preferences icon in the Dock, click the Sound icon, click the Output button, and choose "MOTU 828mk2."
Use aux send 1-2 controls on your source's mixer channel strips to route some of the signal into the SPX990. The SPX990 output returns to a pair of mixer channels.
CAUTION: Be very careful with the aux send 1-2 controls on the SPX990 return channels! If you route the output of the SPX990 back into its input, you'll get ... FEEDBACK! Although this is analog electronic audio feedback, not acoustical feedback, it can still hurt your ears and our speakers. A little bit of feedback, applied carefully in this manner, can sound very interesting, though.
See below for tips on using the SPX990.