The AREF pin is clearly marked on the datasheet. On the Atmega328 in PDIP format it is pin 21.
I'm not sure what they mean by external capacitor, as far as I have seen it isn't usually used.
The analog read is likely to slightly differ from one time to the next. I suggest you average them.
Actually most designs do wire a .1ufd cap from the Aref pin 21 to ground, the Uno rev3 does. It gives a little extra noise filtering directly to whatever internal or external voltage is being selected as the voltage reference for the internal AVR A/D hardware. Don't recall if the AVR datasheet recommends that also or not, but it is generally practiced.