I beat the House Always Wins 1 quest. I went to complete the House Always Wins 2 but for some reason I don't have the Platinum Chip. I know Mr. House gave me the chip. Is there any way to get underground without the Platinum Chip?
I assume you went to the Fort already. There, the guards will strip you of all your weapons and the Platinum Chip. You must now go to Caesar and speak to him. If you are playing along with the Legion, just agree with what Caesar says and he will give you the Chip back, with instructions on what you must do that are different from Mr. House's - once in the bunker it's your call on who you are going to obey.
You can also try killing Caesar and obtain the Chip from his corpse. Good luck on that.