Thanks for using my library. I have a couple of suggestions:
1) try adding ', United States' to the end of the marker position.
2) I believe there is a 'region' parameter you can set on the map. Try setting this to 'US'.
3) if it's still not working, try going into the library and find the get_lat_lon_from_address() function, then output the result of the URL request.
I hope that helps. I'm going to look at adding a debug mode soon which should help with issues like this. Let me know if any other questions though :)
Steve