Depends on your skill level. So your tablet has a 9-pin serial interface?
You may use a max232 chip to convert its 12V signals into 5V TTL for Arduino.
Search max232 chip and download its spec sheet. You will either need some 9-pin connector or cut your tablet connector to hook it to Arduino.
I'd do this:
1) Learn about max232, spec sheet, how to tutorials forum posts
2) Cut the tablet connector or but a mating connector so you can hook it to the max232 chip
3) Feed the 5V side of max232 to your arduino board pins 0 and 1, remove your atmega 328 chip from it.
4) Open the arduino serial dialog and see if the tablet is sending anything that you can read.
5) Use NewSoftSerial (google it) on Arduino board to read positions from tablet
6) Display position on an LCD.
Seem pretty long. Would cost me entire weekend with all parts at hand