hi there
this is my first post out there :)
I think I can amswer your question because I've been there few weeks ago.
it's a bit tricky that I couldn't get there first time.
the road exit to sumaisema is going with a bridge! you will see a bridge crossing the road to Al-Khoor just after a clear sign of Sumaisema. go strait below the bridge (cross from below) and take the first exit after the bridge. this will lead you up to the bridge (you may use it for U turn and will defintely use it when comming back to Doha) then turn left (don't worry .. it's allowed) and go down from the bridge. (making 90 degrees to Alkhoor Main Road heading east).
now go straight to the beach (only 2 KM far from the bridge).

Don't forget to bring everthing you might need from Doha as it's a very small town with few samll stores.

hope you find this useful.
Regards,