My first thought is: "You were supposed to wear (or at least 'to take') short pants, where are they? I don't see them, do you have them in a bag or did you leave them at home?".
Second thought: "Where in your house do you keep your short pants?".
"Where do you wear them" - nope.
"Where in the store can I find them" - maybe. Not with "masz", unless your friend works in the shop so you can be that direct with them. Plural "macie" is more probable, but not exactly polite. "Gdzie mają państwo krótkie spodnie? (Formal You, plural, mixed) is okay. Mixed is most probable because you'd rather refer to the shop rather than some specific people - although if all the staff you can see is of one gender, then perhaps panie/panowie would be fine.