All DS3231 chips have the same 0x68 address.
On the Mega, the I2C pins are 20 and 21. Recent Megas have this indicated on the silkscreen, "SDA 20 SCL 21' is printed on them next to the header.
You do have to have pullup resistors on both SDA and SCL lines, that is part of how I2C works. However, recent megas (or maybe all Megas) have 10K pullups on them already, but it would not hurt for you to add your own pullup resistors to the Chronodot.
If your Mega looks like the one in this picture, then you can just plug the chronodot in to the SDA and SCL lines (and 5v and Gnd too of course) and not worry about adding pullup resistors:http://arduino.cc/en/uploads/Main/Ardui ... _Front.jpg