What does the term mid-supply refer to?
For dual supplies that are symmetric, eg +/- 10v, mid-supply means ground.

When the supply pins of an op-amp are not symmetric about ground, for instance 0v and +5v for a single supply amp, or -4v and +12v for some arbitrary dual supply, then mid-supply means, well, mid-supply, +2.5v in the first example, +4v in the second.

A common way to get an 'RL of 10k connected to mid supply' is to use two 20k resistors, one connected to the -ve supply, one connected to the +ve supply.