I just started doing the Oh My Papa quest. I went to talk to Melissa and she is not in the quarry. The marker shows her there but no one is at the encampment. I had visited her before and did a quest to go to Sloan for her. I did not complete it as she wanted but turned the supplies into the NCR. Does this mean she is just gone and I cannot do Oh My Papa anymore? Or is there some way to make her show up?

Try fastforwarding time by waiting. Sometimes the day/night feature of the game can make NPC's transist from one area of the Mojave to the other. If all else fails, then try looking up more help for it.