NTOP - SPECIFYING INTERFACES

Hi there!

Recently I installed the package NTOP in my pfSense but it´s displaying the data from interface OPT1. As a matter of fact I need to be monitoring the interface WAN and when I tried to switch the interface in sumary -> traffic, the system returned the following message:

Note that the NetFlow and sFlow plugins - if enabled - force -M to be set (i.e. they disable interface merging).
Sorry, you are currently capturing traffic from only a single/dummy interface [rl2].
This interface switch feature is meaningful only when your ntop instance captures traffic from multiple interfaces.
You must specify additional interfaces via the -i command line switch at run time.

I didn´t understand how to execute the procedure mentioned on the last line. Do I need to execute this in the pfSense shell??? Could someone explain better what do I have to do in order add this additional interface, in my case WAN?

I have two gateways, WAN and OPT1 operanting on rl1 and rl2.

I appreciate any help.

Thanks a lot!