In any PSoC3 project, once Debug port is enabled, pins P0 and P0 are used up and are dedicated for debug port. Note that these pins can be used for programming and debugging. Since these pins are now dedicted for debug port, PSoC Creator would not allow you to use these pins as GPIO and hence can not be assigned for I2C communication port. In order to use the USB-I2C bridge to communicate with PSoC3, PSoC3 needs to be configured as an I2C Slave. The I2C Slave Pins have to be pins other than P1 and P1.