I don't know what you made wrong, but when you put a digital input pin onto the topdesign you may connect an isr component to its signal without any errors. Problem is: this is not configurable for falling edge.
Open the pins component datasheet. Search for "interrupt". Do as advised and check the "dedicated interrupt". Connect an isr component to the interrupt output of the pin.
If you place down a "Digital Input Pin", then right-click to open the configuration:
There should be a "Input" tab on the middle-right section; After selecting that, you should see a check-box for "Dedicated Interrupt", turning on the dedicated interrupt should create another wire connection that you can use for the interrupt routine (ISR component).
This helped. When I placed the 'Digital Input Pin', I the symbol was just a pin. But when I go to the input tab and check 'Dedicated Interrupt', it showed as as a component(like a box with pink rectangle on the top).