To process the entire stereo mix, you can connect the compressor as follows (using balanced XLR or 1/4" connections): 
main outs L/R of mixer --> in 1(L) and 2(R) of mdx --> out 1(L) and 2(R) of mdx --> power amp or powered speakers 
This will compress the overall mix coming out of the L/R main outputs. 
Alternatively, you can connect as follows (using a special cable called an "insert" cable): 
mic/line in to mixer mic/line input --> mixer insert out via insert "send" to mdx in channel 1 --> out mdx channel 1 via insert "return" --> back into insert point to be routed to channel and on to main mix... 
This will compress a single channel input by itself, sending the input signal to and from the compressor using a single insert jack and special insert "Y" cable. You can repeat this process on another channel to perform compression on an additional input using channel 2 of the mdx.