You cannot simply turn aftertouch on or off. You have to modify a sound and add AT to it.
Press the SOUND button. Select the sound you want to modify (for instance organ). Press the MENU button. Press the EFFECTS button (on the display). On the first tab select the effect you want to add when aftertouch is applied . You can add two effects to the sound simultaneously FX1 and FX2. (In most cases the sound will have one or two effects already assigned.) Assign the effect to the unused or less important assigned effects (say rotary to FX2). Select tab FX2 where you can fine-tune effect parameters and select aftertouch as the source used (pull-down menu next to src . Save the edited sound as usual.

BTW you can use the aftertouch also as source for other settings when editing the sound parameters.

English is not my native language, so if you want more explanation, just ask or read the manual (download at http://www.korgpa.com/pa_root/pdf/Pa1X/english/Pa1X-252F-ENG.pdf) in particular page 210 and page 329 onward.

Admar