This subject is beaten to death in the Miniplex manual available at http://www.shipmodul.com/downloads/m...x-2Wi_3.13.pdf
Maybe it would help you reading it first and ask the questions about specific OpenCPN (After at least having a look at the FAQ and the manual, if possible) stuff it does not cover later.
The short version of all you will learn by reading the manuals
: Once you are sure your miniplex is connected (Read Miniplex manual) and configured correctly (Read Miniplex manual) and working (They provide the MPXConfig utility with it, just read their docs how to use it, just read the Miniplex manual) simply set the AIS Port in OpenCPN to the COM where your miniplex resides and then GPS
port to "AIS Shared" (Read the OpenCPN manual)