Ah, OK, sorry I misunderstood your question.
AIX's tcpdump will turn it on for you.
It is set dynamically on AIX. The default is off but tcpdump for instance has a default of on, iptrace has a default of off but has a switch for on:
AIX Commands:
tcpdump:
Help -
-p Stops putting the interface into promiscuous mode. Note that the interface might be in promiscuous mode for some other reason; hence, -p cannot be used as an abbreviation for ether host {local-hw-addr} or ether broadcast.
iptrace:
Help -
-e Enables promiscuous mode on network adapters that support this function.
Depending on what you are trying to do, and how you want to do it, you might find some related useful info here:
pSeries and AIX Information Center
Or on any of the above links by searching for promisc in the search box on the left side of the window.
HTH.