A 32.768 kHz crystal is a crystal and needs to be connected to the "Low-frequency
crystal Oscillator" which is connected to the TOSC1 and TOSC2 pins.
Which are the same pins as the XTAL1 and XTAL2 pins!
Are you confused yet?
The function of these pins is defined by the settings defined by the clock selection fuses. These are discussed in section 6.2 of the datasheet.
So you might have to (re)program the "fuses" to allow the 32 kHz crystal to be used.
Also realize that at a 32 kHz clock the MCU will operate quite slowly.
If you want to make a clock using an MCU, consider a dedicated RTC (Real Time clock) module like the DS1307.