The ad blue filling tank is located next to the diesel filler, very easy to do just remember it can cause corrosion and irritation. Should have a counter that you need to reset.
For usage see this website, http://myadblue.co.uk/adblue-citroen/new-berlingo/
There isn't a counter to reset, but warning lights when low. Adblue as stated available at Morrisons £11 for 10ltrs....I have used about 5 litres in 3500 miles but don't know if it was full when new.
Be warned, adblue has a shelf life of about 18 months, check expiry date when buying, so not worth stocking up with it.
Down side of adblue apart from extra cost, fuel tank smaller so range is less. My 2011 one would do 650+miles, the new one, I am starting to panic at 500 miles.
We have found that the counter, or low level warning sign , can take upto 50 miles to reset, so don't panic if it doesn't go off straight away.