FREE, info: http://www.pulseensembletheatre.org or 212-695-1596. As the culminating event of its twentieth season, Pulse Theatre Ensemble, under the direction of Alexa Kelly, will present a Free Shakespeare production of "Macbeth" in Governors Island August 7-8 and Riverbank State Park August 12-28.  The innovative adaptation sets Macbeth as a power hungry Marine, King Duncan as a Warlord co-operating with the US troops and the witches as Afghani war widows.  The tension mounts until Macbeth and Lady Macbeth are brought down by their own greed, guilt and fear.      The production updates the classic by transferring Shakespeare's tragedy from the "fog and filthy air" of Scotland to the murky realms of Afghanistan, where "nothing is but what is not."  The title character, an overambitious Marine officer (suffering from PTSD), is brought down by guilt and paranoia.  Lady Macbeth sings blues and inspires herself with R&B music.  Banquo throws football with his son, Fleance.  The witches are Afghani widows, possibly Taliban or Al Quaeda partisans, who seek revenge for their losses.  Costuming will range from contemporary desert fatigues to Afghan tribal costumes. There is a multi-ethnic, Equity cast of 16.