I have trouble too with almost same setup, except the LCD is the simple Blue version. I get a different error message, I'm sure a different problem. I was using this Pi Plate with the Internet Radio project, but had to stop because of trouble. I decided to try the Pi Plate as a standard setup to test the LCD plus keypad combination. The error I receive is this, the same I get with Internet Radio project. So, can't test the LCD Pi Plate with both projects.
Traceback (most recent call last):
File "Adafruit_CharLCDPlate.py", line 442, in <module>
lcd = Adafruit_CharLCDPlate()
File "Adafruit_CharLCDPlate.py", line 93, in __init__
self.i2c.address, self.MCP23017_IOCON_BANK1, 0)
IOError: [Errno 5] Input/output error