Where is the OBD port for my 91 Ford Thunderbird?

its located under the hood it should a black triangular shaped cover

Your car is to old to have a OBD II connection under the dash. Yours will have a OBD connector under the hood on one of the fenders near the hood hinges or on the firewall. It should have DIAG. written on top of the connector holder. unplug the to connector under that and attach the scanner tool there. The scanner will need to have a OBD I cable to hook up.