This post is a follow on to this one.
It is a little unclear which pin on the BAS120 printer module is which. In the images below, RX/TX is white, 5V is red and 0V (gnd) is black.
The easiest way to connect an Arduino board to the module is to solder jumper wires to the three holes (which connect to RX, 0V (gnd) and 5V) as shown below.
These three holes appear incorrectly labeled from the front of the module (red side) because the text refers to the header that is normally used to connect to a basic stamp. The back side (green side) of the module correctly labels these pins (shown in red in the second picture below).
The TX pin is marked on the Arduino as TX. It is pin 1 (the second pin from the bottom) of the digital pins.