Hover your mouse over the rig name and the ip is in that box.
Just keep in mind generally if you reduce the payout below 1 eth they (the pool) take a fee
I think your public IP address (NOT your rig IP) is required to do this.
As long as you are in same domain/network where your rig mining..... just launch browser and google "whatsmyip"
Edit: is this is dwarfpool ? If yes, they waiving fees until June 4th because of some payment delays/issues...