This circuit is an expanded version of the FatMan External Input. Really, a mixer is just a collection of preamps that all tie together at a common current summing node.
The input and gain resistors (Rg and Ri) have no values in the drawing because you might want to optimize these values to whatever that channel will amplify. Low level signals will require more gain than high level ones. Ri is used to set the imput impedance of the channel while Rg sets the gain. To set a gain of A, Rg must be A times larger than Ri.
Master Gain Trim is a panel control that you use to eliminate distortion due to clipping at the VCF input.