Do I need a foundation for this type of shed, or can I just put it on level ground?
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.
I got pea gravel and spread it onto bare ground after I raked it. I have had my shed for 4 years. It is not sinking. YMMV but I don't think you need to lay a concrete surface. On older, wood sheds it was recommended due to rot, but in a Rubbermaid shed I don't think it is necessary.