havnt used the os, but just a thought. make sure the samples are cut up properly, i.e 4/4 time sig, 1 bar, start/end points precisely set. click on that detect tempo function and it should calculate the correct bpm, then stretch inputting what the original bpm is, and what the new bpm u want it to be.
dang wish i had the os!! they havnt decided whether or not my mpc has been lost during its delivery to the repairs centre or whether or not its lying on the floor in a warehouse somewhere. fingers crossed, the delivery company has lost it and can get a new one on insurance! i just wanna use these new features. otherwise im gonna hav to wait about 2 months to get it back and also pay shit loads on repairs. tho my luck is kinda looking up, looks like i got paid about 600 bucks more than i was supposed to get paid. sorry to go off topic, just gloating