The black plastic 
cowl is the piece that lays across the base of the windshield.  It is accessed by opening the hood.  There you will see the plastic hold down push pins(rivets) that hold the 
cowl in place.  Look carefully at the rivets you will see a small inner circle in the rivet top pushing that inner circle in will release the outer top & the complete rivet will release.  Just pull & wiggle it out of the 
cowl.
The 
cabin air filter which you are referring to as the 
a/c filter is in its own "box" under the 
cowl in the corner of the passenger side just under the windshield.
You do not need to release all of the 
cowl push pins to access the 
cabin filter, but do release & remove at least half way across starting at the pass. side. You do not need to remove the wiper arms.  Once the 
cowl is loose you can lift it enough to open the 
cabin filter's box & pull the old filter out.  Install the new filter, can only go in one way due to its shape.  Latch the filter box closed.  Press the 
cowl back in place paying GREAT attention to the rubber windshield seal.  It MUST lay flat against the glass to create its seal.  Otherwise you will have wind noise as it flutters as well as water leaks.
Once the 
cowl is down in place put the rivets back in latching them by pressing the center pin back down flush with the outer ring of the rivet.
If you break one or loose one down in never-never land they can be found on-line by searching Ford's 
part # W705956-S300.
Depending the the dirt/dust level where you live it is a good idea to change out the 
cabin filter at least once a year, for what they cost & the problems they prevent, twice a year is even better.  Just like you house/home furnace filter.