The 5 plugs on the left hand side are for each of the 5 stepper motors. The bottom 3 are for X, Y and Z, in that order, reading from the bottom, so the top two are for your extruders. You'll want to connect the Discov3ry motor connections to one of the upper two. I'm not familiar with the connections to the Discov3ry but I'm sure it'll have 4 stepper motor connections. Which connections are which won't really matter much. If you've got them the wrong way round, the motor will simply rotate the wrong way so you can change the connections around until it goes in the right direction.
If there are any other connections, I'd just expect them to be for a microswitch or some such to detect when the syringe is empty.That would probably connect to the connector labelled "EXTRA" on the upper right hand side.
Pure guess work but I'm pretty confident it'll be right.