Just type the information. Do NOT capitalise the elements (i.e. enter sn NOT Sn for tin). FX ChemStruct will do everything else. There are a few tricks to learn, but they should only take you a minute or so to grasp.
Perhaps the major "trick" to entering structures is that you must give FX ChemStruct some structural information as you are entering the structure.
For example, if you are trying to draw propane, do NOT enter c3h8. FX ChemStruct will take this information and try to build a structure out of it but will get confused as it is ambiguous.
Instead enter propane as ch3ch2ch3. This tells FX ChemStruct WHERE to place each hydrogen.
There is much more information regarding how to enter different structures later in the help information.