Do plastic sheds require a foundation?
The assembly instructions state:


  Recommended surfaces for installation include cement or treated wood deck style surfaces.


So it should be installed on a level platform of some kind that will support the weight of the shed and its contents.

It wouldn't be a good idea just to use level ground as you'll have problems with moisture, rodents etc (as Eric Petroelje points out in his comment). You may well also find that the shed will sink unevenly into the ground making it difficult, if not impossible to use.