Now that you mention it, request forms like `してください` are not so frequently used in tooltip text.

> I wonder if it should be `…を指定` or `…を指定する`

I think it should be better to use the polite form `…を指定します` here since this string apperes only in tooltips.
 * If the string apperes in lables of menu items/buttons or tooltips of icon buttons, it should be translated as `…を指定`. The reason is likely that a user clicks a widget like that __to order the program__ to do something.
 * If the string apperes in labels of radio buttons/check boxes, it should be translated as `…を指定` or `…を指定する`. Many labels like that have longer width than buttons.

It is, however, difficult to differentiate messages by their role strictly since in gettext `msgid` is usually the same as original text, though.