Kab,
-The R18 connection goes to R18 on the board. You have to reference Rob's schematic to find this. 

-The inputs are banana jacks which are ungrounded. They don't need grounding if you are connecting to points within the same circuit. If you connect to a different circuit you have to make sure the ground is connected for each circuit. The point of a ground connection on a jack is to connect the ground point on different circuits that are plugging together. Its not inherently necessary for audio or voltage patching, just for circuit compatibility.

-the 2 position switch is a SPDT switch (single pole, double throw). That means the center pin connects to one of the outer pins depending on which way the lever is thrown.