Per Dell's specification page on this display:
Optimal resolution:
1920 x 1080 at 60Hz
Important question that you haven't yet answered:
What kind of cable are you using to connect the monitor to the MacBook?
Also, check the display's input settings.
Although I've never experienced the problem, I've seen a number of postings here at MacRumors from folks who had Dell displays that wouldn't connect right because the Mac was trying to sync to them "as tv's", rather than as a "computer display".
There is a workaround for that...