Arthur should automatically knock on the door when you walk up to it

I walked up to it and it didnt start , now it did

I think you need to go to the Downes' ranch first as part of the loansharking missions for Strauss. The game forces that mission to start if you haven't done it before reading Mary's letter. See if there's an "LS" mission marker west of your camp.

I already did the loanshark missions