The CurrentKeeper should be connected to the decoder ground (V-) and the Function Common (V+).  As you can see in this illustration there is connection on the decoder for each connections for each:

  1. You will need to be very careful soldering since you will be soldering to the parts on the PCB directly, so a fine point tip and proper tools and supplies will be needed
  2. Make sure that the flux you use is designed for work on fine electronics.  Using the wrong flux can cause damage to the decoder and the parts
  3. Use small amounts of solder as to prevent any solder bridges that could occur with these parts being so close together