Specify the interface you want to use in "/ip traffic-flow" then specify where you want to send the traffic-flow information in "/ip traffic-flow target"
But I need to set type of accounting traffic (from user, to user or both) on the interface. For example, I want to see only traffic TO user on the specified interface. In Cisco IOS I can set "ip flow egress" or "ip flow ingress". How can I do that in RouterOS ?