On the switch board, the "neg" goes to the relevant pin labelled "ground" on the picture PB posted. The switch board "ground" goes to signal ground, and to the TRS jack ground (shield). "pos" goes to the tip or ring on the L+ or R+ on the XLR, AND to the tip or ring on the TRS.
The shield of the XLR cable (assuming there is one) should go to signal ground as well - this may or may not be important for protection from radio frequency interference.
Connected this way, you will have sound available from XLR and from TRS, but setting the "bal/unbal" switch correctly gives optimal results for the chosen application.
Hope that helps!