Problem with reaching fortification hill in fall out new vegas?

I cant seem to reach fortification hill. Well im towards the end of the NCR quest line, actually on the last quest before the hoover dam battle and i need to reach fortifacation hill to get the quest from my companion arcade gannon. Does anyone know any other way to reaching this place with out first going there when invited from caeser him self or am i pretty much screwed now. Also if theres any alternate way to trigger the quest from gannon please if you know let me know. Thank you.