Ok, I see what happened ...

The address listed in the datasheet is only 7-bits 1010100.
When you add the read/write bit it becomes $A8, 1010:1000

And I think the pointer address should be sent with I2CWRITE before an I2CREAD, instead of in the I2CREAD.
Yes it's default to 0, but this should work for all registers ...
Code:
slave_address=$A8
adc_register=0
I2CWRITE i2cdata,i2cclock,slave_address,[adc_register]
I2Cread i2cdata,i2cclock,slave_address,[data_read.Byte0, data_read.Byte1],error
hth,