The stats needed for the calculator can be found on your Stats window in Maplestory (circled in the following image)-
The Spell attack can be found in your skill list or with with information such as this -
Note: If a spell has no Mastery listed, then the mastery should be 60%. (Blizzard, for example.)
Change the remaining values according to your skills and equipment.
If you have Elemental Amplification skill, choose the appropriate level in the drop box.
If you are using an Elemental Wand or Staff, select the bonus that applies to the spell you are calculating. (Remember, spells with mixed elements, such as Composition, do not gain any EWand bonus.)