Well, depending on what software you're using in your computer for a library of sounds....
1) Set the Octapad to transmit on a channel (1-16, but traditionally, drum instruments have used MIDI channel 10).
2) Set your computer software to receive on MIDI channel 10, now the two devices are talking to each other (or the computer is 'hearing' the octapad come in on channel 10
3) The sounds you're using in your computer, must correspond to MIDI note number (this will be a number between 0 and 127). So however you do that, pick a sound and assign it to a MIDI note number (for instance, your bass drum sound will land on MIDI note number 35, snare on note number 38, etc.,....)
4) Select a pad on the Octapad (I think you're in edit mode on the Octapad), hit it, and select the Note Number function. At this point you can assign that pad to the corresponding MIDI note number of the sound you want to trigger in your computer.
5) Voila! Now your pad will trigger that sound. Repeat the process of assigning note numbers to pads and sound in the computer to note numbers and you're all set.
NOTE: These days, since there are so many different sounds in a sound module, it is easier to create separate drum kits in the module (like a person would if he had V-Drum sound module, where there can be as many as 100 different drum sets). By using this method, you can create different sets, but you're using the same MIDI note number for each different snare sound (or bass, or tom, etc.,...) Then you can leave the Octapad alone, and just change drumsets by selecting different drum sets from your computer. This allows you to have alot of different sounds, because you're only given 127 note numbers to begin with. There's no way you can assign thousands of sounds with only 127 places to put them!
Good luck!