I think you are refering to a E90 model 325i and if you cannot find a dip stick under the bonnet BMW have removed the dip stick and you can check the oil electronically in 2 ways depending on the spec of your vehicle.
1.if you have a car with a i drive system the oil level can be checked via the central display using the i drive system.press and hold down the i drive controller and go to the service menu next to this you should see a i con showing oil level check click on this and follow the instructions given.
2.if you do not have a i drive system then you can check the oil level via the left hand indicator stalk,using the knurled wheel and the press in button on the end of this stalk.
using the knurled wheel keep flicking down one by one until you see a picture of a oil can in the instrument cluster LCD display once you see the oil can press the button on the end of the stalk and this will display and electronic picture of the dipstick showing the current oil level.
before doing both of these procedures run engine up to temp and park on flat area and leave at idle for 2 mins and then start procedure to get accurate reading.if you are still struggling please check owners manual as this info is all in the owners manual under oil level check.