HI,
I don't see your code but a common mistake is to think that the slave address is expressed as a byte: it is a 7 bit word, which should be followed by the direction bit (0=write, 1=read). In your case, 0x68 gives the 7 first bits of the byte. You should then send 0xD0 to write something to the device, and 0xD1 to read something (this assuming your device is configured with pin 9 low, if not the 7 bit adress will be 0x69). Hope this helps.