It was added in the second printing. An errata update is in the works to add it to there as well. In the mean time:
TASER M/BTAS#
Record this as M/BTAS# where M indicates a ’Mech taser, B
indicates a battle armor taser, and # is the number of tasers
mounted on the ’Mech or battle armor.
In addition to any normal attacks allowed the ’Mech each
turn, it may make that number of taser attacks divided among
any number of targets within firing arc and short range. Battle
armor Elements with tasers may make that number of attacks
per game; i.e. a squad of battle armor with 4 tasers could make
4 taser attacks per game, while a ’Mech equipped with 2 tasers
could make 2 taser attacks every turn. The effects of a successful
taser attack are modified from Tactical Operations (see p. 347,
TO) as follows: the attack causes no damage and the duration of
the effect (whether shutdown or interference) is always 1 turn.
Conversion: ’Mech or battle armor taser