Site for greenhouse:

You don't need to put the greenhouse on a cement pad or footing even if your in a 
frost zone. The Lexan will flex if the ground under the greenhouse heaves and will not 
break like glass or plexiglass. A glass greenhouse should have a cement foundation. 


Anyway...

Clear a site one foot wider and longer than the dimensions of the greenhouse and dig down
about 6 inches to clear away any sod or roots. Level...add 4 inches of sandy gravel
(called 3/4 down) level and tamp down.
After it's packed and level, lay down black landscaping material to keep out unwanted plants. 
Set the greenhouse down and fill the remainder on the inside and out with decorative stone 
(pea gravel, limestone rock). The greenhouse foundation consists of a 2x4 pressure treated footing,
a cedar 4x4 then a cedar2x4 sill (6 1/2 " high in total)

The pressure treated footing will not be visible on the inside or out. 

In windy areas you can run a "T" piece of rebar thru the foundation into the ground at each corner.