I don't know if you were able to solve your problem or not, but I found I needed to change the busnum as instructed in the tutorial for the newer raspberry pi model B, and also in this line of the Adafruit_CharLCDPlate.py code.
lcd = Adafruit_CharLCDPlate(busnum = 1)
When I did that, it would work, but only after I had run the program once. To get it to work right after booting, I also needed to change the busnum in Adafruit_MCP230xx.py in a couple of places (basically look for anywhere busnum=0 and replace it with 1), and in Adafruit_I2C I replaced the little bit of code that's supposed to autodetect which busnumber to use with '1'.
I hope this helps.