The Kaoss Pads are basically effects processors - they have an input and an output...
so what you do is you take the stereo audio output from your synth or record deck preamp or guitar preamp or whatever, connect that to the stereo IN (left and right in) connectors on your KP1, and then connect the output from the KP1 to wherever the source was connected before you added the KP1.
for example:
Synth > Mixer
becomes:
Synth > KP3 > Mixer
Current Gear: Kronos 61, RADIAS-R, Volca Bass, ESX-1, microKorg, MS2000B, R3, Kaossilator Pro +, MiniKP, AX3000B, nanoKontrol, nanoPad MK II,
Other Mfgrs: Moog Sub37, Roland Boutique JX03, Novation MiniNova, Akai APC40, MOTU MIDI TimePiece 2, ART Pro VLA, Focusrite Saffire Pro 40.
Past Gear: Korg Karma, TR61, Poly800, EA-1, ER-1, ES-1, Kawai K1, Novation ReMote37SL, Boss GT-6B
Software: NI Komplete 10 Ultimate, Arturia V Collection, Ableton Live 9. Apple OSX El Capitan on 15" MacBook Pro