Try to install any other IVI-COM drivers such as from Agilent, to check if they appear in NI-MAX. If other drivers correctly appear in NI-MAX, it may be a driver-specific problem of Anritsu driver due to such as incorrect registration to IVI Configuration Store.
Also what NI-MAX version are you using? Older NI-MAX versions do not check IVI Configuration Store and then IVI-COM drivers are not shown.
