You mean this model, right?
http://www.yamahaproaudio.com/products/mixers/mg_cx_models/index.html

I've never heard the MG124cx, but here's what I can tell you by taking a quick glance at the manuals (I like their PM4K, though)...

Pretty standard mini mixer with built in FX.
You would limit it's capabilities using a BR-864 because the 864 appears to record a max of 2 tracks simultaneously.

The CD player you'd want to plug into the 124's stereo tracks 9/10 or 11/12 via the 1/4" jacks.

The insert outs on the 124 are phase flipped, something to keep in mind.

Here are those manuals:

124 manual: http://www2.yamaha.co.jp/manual/pdf/pa/english/mixers/mg124cx_en_om.pdf

864 manual:
http://lib.roland.co.jp/manual/en/dl_04-00241/BR-864_e1.pdf

thanks, here the thing when i plugged my cd player in the way the book says i get nothing. same with the br 864? dont get it must not be doing something right thanks for the help

That is odd.
Does the CD Player have gain pots on the back (they might be accessed through holes in the chassis).

What model CD player is it?

What type of cables are you using to hook it up to the gear?