You will need to install it again. Or edit descr_formations_ai.txt and add this to the end:
Code:
;****************************************************************
; Alternate Ambush deployment (required with Alexander exe)
; Column marching formation for the army being ambushed, allows stretching
;****************************************************************
begin_formation alternate_ambush_defender_deployment
alternate_ambush_defender_deployment
ai_priority 0.1
;;; general at the front
begin_block 0
min_units 1
max_units 1
unit_type general_unit 1.0
unit_density close
block_formation line
block_relative_pos 0 0.0 0.0
inter_unit_spacing 2.0
priority 1.0
end_block
;;; all other units behind
begin_block 1
unit_type any 1.0
unit_density close
block_formation column
block_relative_pos 0 0.0 -2.0
inter_unit_spacing 2.0
priority 1.0
end_block
end_formation
This IS indeed the only difference between ALX and BI installations, other than the Imperial Campaign and Shortcut.