If you look at datatsheet there are two muxes, controled by registers,
that feed the + and - input to the ADC. Look at example code at bottom
for how the pin is connected to ADC via register write.
From datasheet this is why you do not "see" it -
The ADCINC User Module consumes the SPC block, but this block is not displayed in the Chip Editor. Only one instance of the ADCINC User Module can be placed because one SPC block is available in the chips, which support this user module.
Thanks for the reply :)
I am trying to design my circuit now. I am basically using SPIM and WUSB and timers. I am not using ADC its just for example that I spoke about it. Yes, I got the idea why pins are missing on modules.