Don't try and find it yourself find, some people that now the way and tag along. A common meeting point is Airport Petrol Station. The QNHG also organise trips down.
Do a Google image search for Qatar Inland Sea and you'll find it.
To go to inland sea you will need the GPS ... from sea line resort will take about 45 to 60 minutes off road to reach there. It is west / south west - ish direction from sea line resort.
Have a look at the photo gallery i think i saw some pictures from inland sea there.