ok first of all.. use the search, because there are hundreds of threads related to sampling different things into you mpc.
now secondly.. remember this cause it is important. Sound does NOT go through midi cables. all midi cables do basically is send commands. picture it like this, you have a midi cable inside your mpc connecting the pads to the part of the machine that plays the sounds. so now when you connect the midi cable from the mpc to something else, you are using the pads on the mpc to play sounds on other machines. ... it is sorta like the wire that hooks your computer keyboard to the tower. it will make sense over time, once you try out some more stuff with your equipment.
now what you need is a wire that on one end it has a 1/8 inch jack (headphone jack) and on the other end the wire has a red and a white RCA jack (playstation jacks.. but not the yellow one, only red and white.)
you put the headphone end of the wire into the computer, and the red and white goes into the mpc (you will notice that they wont fit, but you get rca to 1/4" adapters for them.. here are some pics for you anyways)

and at the ends of the red and the white jacks, you need these:

......
however, if you have a good electronics shop, you can cut to the chase and just get this:
1/8" stereo to 1/4" mono adapter

except I find those are a bit harder to find, but every radio shack (circuit city) has the first two ones....
how it's like your computer is an infinate sample source!