Hi,
I think the best way to do this would be to create a second vSwitch, then assign one of the physical NICs (can figure out using the MAC address) to the new vSwitch. (You will plug the internet connection to this port)
Next create a VM Network under that new vSwitch and call it something like 'WAN' or 'untrusted'
Next create your VM for pfsense, give it two NICs the first one set the the VM Network from the original vSwitch (your LAN), the second the VM Network from the second vSwitch (your WAN)
Be sure to note the virtual MAC address so you can assign the correct interface within pfsense's install.
Now I would re-IP the service console to an internal range address (note: you will likely want to be onsite for all of this)
Give the pfsense firewall LAN address an IP and access the web gui. Input your WAN information (if it is a static IP). Plug in the WAN cable. Now set the gateway of your other VMs to use pfsense.
Hopefully that makes sense, I don't know what all your requirements are but I have a similiar setup running (1esx server running pfsense as a firewall for another VM and multiple physical machines.)