With the information you have given I would say get sure
-that the sound starts right at the beginning of the sample (that there is no silence at the beginning),
-that the sample is long enough (I think it has to be longer than 800 sample points to be audible),
-that you are playing the right program (DRUM 1 - 4, right program number) and the right note number,
-that the pad is not told to mute itself (this can be changed by selecting the NOTE OVERLAP parameter and hitting the OPEN WINDOW button),
-that the Attack/Decay envelope is set right (better set Attack to "0", Decay to"5", Decay type to "END"),
-that the pad is not playing velocity dependant (this can be changed by selecting the ATTACK, or DECAY parameter and hitting OPEN WINDOW,
the amount of different influences that change the sound of the pad´s sample may be edited there, better set all to "0"),
-that you play the sample with enough velocity (better with the FULL LEVEL key switched on),
-that the filter cutoff amount is set to 100
-that the stereo mixer settings for the pad allow the sample to be heard (volume up, PAN set to MID, FOLLOW STEREO set to NO),
-that the effect mixer allows the unprocessed signal to flow to the stereo outputs (better don´t assign the pad to a FX unit at first),
-that the track which you are playing in is not muted...
...this should help you keeping busy
