By "flat panel", I assume you mean an flat unstiffened panel like a flat metallic plate or a bare laminate, is that right?
The concept that I think you want is in the Unstiffened/Sandwich Panel Family and is called "One-Stack Unstiffened". Choose this concept and put in a material and thickness range for the "Stack 1/ Top Face" variable and you should be off and running.
What might be a little confusing is the way in which sandwiches and unstiffened plates are related. The concept families in HyperSizer, such as "Unstiffened Plate/Sandwich Panel Family" are meant to let you put a bunch of different panel types in a hopper and let HyperSizer choose the optimum concept. Therefore the variables tab does not contain variables for just one panel concept, but for ALL panel concepts within a family.
So, even though you want an unstiffened plate, the variables tab shows you variables for one-stack, two-stack, and sandwich. If you want to design a one-stack, you simply need to put in materials and ranges for the top stack, and choose the appropriate concept on the concepts tab (one-stack unstiffened), turn off the other concepts, and HyperSizer will ignore anything entered for the core or the bottom face variables. Let me know if this needs further clarification.
