I've just spent an hour scratching my head as to why I couldn't get SPI Slave working on an ATMEGA3208
The 0-Series manual states this for the SPI registers:
But the register at offset 0x02 is not reserved - it is the INTCTRL register, and you need to enable the individual register bits to enable the interrupts!
It is defined in iom3208.h as SPI0_INTCTRL
I finally found this error/omission in the manual by creating an Atmel Start project for the XMEGA3208 and enabling SPI as slave, then comparing the setup code to mine.