How do we access the USO from the terminals? Is there an under ground tram or tunnel? Or would we have to walk outside and across the streets?

The USO is outside security in the building that looks like a spaceship. The entrance is facing terminal 6. The terminals at LAX are in the shape of a U with terminals 1 an 7 forming the tops of the U shape and Bradley International Terminal forming the base. The USO is between 2 and 6. It is closer to 6 with the entrance facing 6. It is only a few minutes walk (or shuttle ride) from the entrance to any terminal... the longest being from Bradley.

Walk across is the only way.

If you are in a terminal that is farther from it, you can use the airport shuttles to move around the loop. But, eventually, you have to leave the terminals, cross the wide and busy street, and walk across a parking lot...

As far as I know, you have to walk the streets. There are little carts that sometimes go by and pick people up but I believe they're for disabled people.