To create instances of elements, drag them from the tool panel on the right to the workspace. Elements display a rounded decimal approximation of their value. However, when elements are stacked, the approximation is red, and only the top stacked number is displayed.
Drag and drop an element onto another to combine their values on the stack. See Wikipedia for more information on stacks.
To use operators, drop them on top of the proper element in the workspace. Unary operators (L and J) work on the top stack item. Binary operators, including addition and subtraction, operate on the top two stack elements