Glossary: Aftertouch | Sweetwater.com
in studio one, when you open any plugin or instrument, towards the top left it will list the name of your midi controller. to the left of that is a little symbol that looks like a gear you'd find in a vintage watch/clock. click that and two black rectangles will appear below it. now click on the knob or slider on the instrument/plugin you want to automate, then drag your mpd slider. now click the "link" icon between the two black boxes and your slider will be connected to whatever knob/slider you clicked on earlier.
on a side note, both of these questions could've been answered yourself with 2 seconds of searching google and your Studio One manual.