atmega2560 i2c problems
A common problem when using I2C is to forget about shifting the slave address one bit to the left before using it.

An I2C address is just 7-bits left justified with the R/W bit at the bottom.

You don't say what the slave device is?  Are you sure the slave address is correct.

I haven't written i2c code for the atmega2560 but I have for other AVR family members.  The last one I did for an atmega328 I used the library from Fleury I2C library.

I found it useful to write an I2C scanner that would attempt to read from all i2c addresses and print out the ones where it received an ACK.