drivers side of the oil pan towards the rear of the pan.
when you jack the car up, stick your head in behind the drivers side front tire and you should plainly see the sensor about midway up the oil pan.
Are you getting a "constant" check oil level warning or is it only intermittant.
If it's intermittant it's probably because you are "close" to being low on oil and there is still 1/2-3/4 a quart of oil up in the heads after shutting down.
Mine will do it.. intermittantly, it means I'm about a 1/2 a quart low.
I ignore it until it comes on more than just "once in a while".