Looks like you reviewed the M103 coolant change instructions in the manual/CD-Rom. The 14mm bolt(very short) that you'll refill thru is the one at the top of your photo. Back it out slowly and make sure you get it threaded correctly when reinstalling. These are easier to strip than you might think.
I also remove the t-stat housing and t-stat first, then start refilling thru the hole that's exposed once the 14mm bolt is out. You'll see coolant rise up to the top of the t-stat housing. This technique helps eliminate the trapped air issue. Once the coolant is up to the top of the t-stat housing, I install a new t-stat and gasket, then reinstall the t-stat housing cover(3 small bolts). After that I add coolant thru the expansion tank until it fills up not all the way, but close to the top, then I reinstall the 14mm plug. When you start the car, the level in the exp. tank will drop some after the system begins to warm up. I leave the exp. tank pressure cap off for awhile to help ensure any trapped air is bleed off. In one old thread, it was suggested to elevate the front of the vehicle to help expel trapped air.
I personally prefer to premix my coolant and H20 before starting.
Take a clean, empty one gallon jug. Pour in 64 oz. of new coolant, then add 64 oz. of H2O. After that, add 64 oz. of H2O to the source coolant jug, put the caps on both and shake them up.