Step 1: Charley's Experience.
Charley could never again find "The Third Level" because it was a place that existed in his imagination, a result of his desire to escape the stresses of modern life. Despite trying repeatedly, Charley was unable to locate it again, as it was a manifestation of his subconscious mind and not a physical place.
Step 2: Conclusion.
Thus, Charley's inability to find "The Third Level" illustrates how his search for peace was more internal and psychological than something tangible in the real world.