How to relay an emergency to ATC while on the ground?
Call the police if in doubt who to call they have contact with Air Traffic Control. 

Also calling the wrong facility can mean that it can take quite a while for the information to make it to correct facility.

Finding direct line phone numbers to ATC facilities can be intentionally hard. Prank call anyone! They are usually somewhere in the local Aeronautical Publication Books but where you can get a copy of them and then finding the number inside the book is difficult. Just call the police!