I tried the donut basemant but I didnt do that quest yet.

where is the sewer entrance anyone got screenshots or map locations?

You can enter the SDM from the back of the building through the Receiving Dock area. (Through one of the large garage doors.)

there is an under area that needs to be accessed using a sewer though Im reading online, I cant find this sewer and the wasteland is to huge to ever stumble across it. Have you found the sewer entrance?

Have you tried the front door?

You're thinking of the Switchboard. That has a front entrance under the Slocum's Joe and a sewer entrance Northwest of the Corvega Assembly Plant. The Super Duper Mart, on the other hand, is pretty much like it was in FO3: a little side area you can go into and explore, but not vital to much of anything. The one in this game even lacks the quest hooks the one in FO3 had.

That's how I got in. Seemed pretty logical.

When I arrived (as part of a quest) there was a woman asking me to help her friend inside. The front door was open but apparently we were supposed to come in the back door!