Trying to Locate the Common or C Connector
The C wire should be the grounded terminal on the transformer. 



You can confirm this by measuring AC voltage between the R terminal on the board, and the grounded terminal of the transformer.  You should measure somewhere around 24 volts.

Inside the unit, look for the place where a bunch of green wires meet up with a white wire, and attach to the frame of the unit.  Follow the white wire back to the transformer.   Attach you new C wire, to a convenient place along that path.