Is it accesible on foot or air train from the international terminals?

The USO is only accessible by foot from any terminal. You must leave the terminal and walk over.

Walt's answer is correct. BTW, there is no air train at LAX.

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.