As suggested, you may need pullup's - I2C won't work without it they 'may' be fitted on the Uno board?, or internal).
However, I'm always rather confused why people want to use I2C to communicate between two micro's, there are MUCH easier ways, a simple serial (RS232 style) connection is always my preferred method. I2C was specifically designed to interconnect multiple IC's on PCB's, in order to reduce the number of required connections between them all - not to talk between two micros.