Why aren't passengers instructed how to lift aisle armrests?
The FAA provides some useful links in their cabin safety index document here. It seems according to this document, as well as this bulletin arm resets (in general) may actually provide a bigger safety risk when lifted and that they should be placed in the down position for takeoff and landing for emergency reasons:  


  3-3484 PROTRUDING PASSENGER SEAT ARMRESTS. Inspection of the Hardman Model 9500 and other passenger seats installed on several
  aircraft disclosed that the armrest in the upright or stowed position
  can protrude approximately 45 degrees aft the seatback. In the event
  of an emergency evacuation, protruding armrests could present an
  obstacle between seat passageways, obstructing emergency exit access.
  Air carriers should emphasize to F/As that prior to takeoff and
  landing they verify that the armrests are in the normal forward/down
  position in order to ensure that they do not obstruct the passageway
  between the row of seats leading from the aisle to the emergency exit.


Also it would seem that according to §382.61   What are the requirements for movable aisle armrests? the movable arm rests are mainly there to allow the easier seating of those with limited mobility and not intended to aid in evacuation.