When and where to use a compressor?

You probably want to invest in a patchbay. This is the little unit where you patch all of the I/O of your device do and therefore will able to poke your compressor anywhere you want in the signal chain.[/color]

you can't resample back into your MPC but you can of course sample it or record it onto something else and then bounce back. I normally actually sample through my compressor in the first place to 'level out' samples & get them as loud as possible.
If you want your drums to pump loudly put your attack high and release short. and tweak ratios and thresholds to get what you want.
I would put the compressor across you stereo outs too which is helpful on the whole mix, if you rcompressotrs stereo.

have fun and be careful that the compressors just not making it louder rather than re0-shaping.
808 kick are good through compressors for giving the body off the boom more presence...

just keep experimenting, it's taken me years to fully know what i'm doing with my compressor

lex.
PS you can sample back into the 4000, but you don't wanna know that if you don't have one[/color]

What patchbay do you recommend? How much? Doesn't patchbays keep you from having cables everywhere. That's also a problem.[/color]

i use an ultrapatch
works a treat.
like the man says you do need alot of cables though, which in turn is bad for noise. I used to just permanently have two channels through my compressor all the time 1&2 for kick/snare separately or stereo it up for a stereo break. this meant i kept all my cable runs short.
patchbays are great if you've out grown you mixer too, run out of sents/inserts etc. I use mine all the time to send stuff all over the place, great if you want to use the sidechains on your compressor too, you can use these to compress your bassline when your kick drum kicks therefore giving it  abit more space.

lex[/color]