right on the top of the box, just where the selector enters, there is a plastic mushroom cap, prize that off and a breather is revelaed, then use a deep socket to remove this breather, this then unveils the filler hole.
The F20 holds roughly 1.8 litres, i still managed to put 2 litres in mine, but that was a complete dry fill.
If your going for a full oil change, drop the diff sump plate, you will only need a 1 litre catch container cause no matter how hard you try you always end up with a litre over the road/drive
You need 80EP or EP80 manual transmission oil, depending which way round the maufacturer has muddled the code. I used comma hypoid synthetic EP80. Its red, and smells nnnnnnnnice. There was some castrol stuff in halfords marked up 80w90, dont think this is suitable, it has to be EP80.