A few ways
1. Use the riverboat tickets.
2. Complete The Local Flavor (possibly) or up to the part where you meet that missing girl.
3. Complete all Main quests of Point lookout.
Note: i played Point lookout like twice then stopped playing for almost 3 months since i got tons of new games.
Chat with our AI personalities
If you already have the dlc for it you go to the river boat landing. If you don't have the dlc you cannot go there. P.S the river boat landing is by Rivet City .
Point Lookout is on the southern tip of what used to be Maryland. To get there, you need to travel to the far south of the Capital Wasteland map, to the Riverboat Landing location, and get on the boat.