Mark,
That is correct. B5 configuration does not have that command enabled for the ports like the B3 has. Setting that command should bring the LAG up. You can also verify if LACP packets are being broadcast every 30 seconds on ports with command "show port lacp port ge.1.46-48 counters" and look at the LACPDU TX and RX values for transmitted and received.
Regards,
Ryan Careno