You should have written the .ISO image as a "data image" onto your USB drive from your PC first.
Doesn't matter, you don't need to do this. You can do this from IPMI.
Have you managed to get the IPMI page up yet? If the board's green IPMI light is flashing (it should be as long as the power supply is connected to the wall) you should be able to pull up the IPMI web page. Your motherboard manual has instructions regarding how to identify which IP address your IPMI page is displayed.
On the first IPMI page, there should be buttons across the bottom left. If your server is off, it will indicate "host is off". Press the "Power On" button. After a few seconds, the window above these buttons - the "remote console preview" window will start to show the bootup process. Click on this window. After the Java applet loads, you will now have a window which is the video output of your remote server. (Isn't that cool?)
It's probably stuck now, but no matter. The Java iKVM Viewer window has a "Virtual Media" menu on the left. Click on it, then select "Virtual Storage". In "Logical Drive Type" select "ISO File". On the button to the right select "Open ISO" and browse to the FreeNAS ISO. Then select "Plug In". The Connection Status History should read "Device1: VM Plug-In OK!!" Press the OK button.
Back in your Java iKVM Viewer window, select "Power Control", "Set Power Reset". Your server will reset. Pay special attention to the text, at a certain point you should see "Select F11 for Boot Options" or something like that. Press F11 and select the "CD-ROM".