Vera says to Ivan that she has already been to the exhibition:
Да я ужеходила на прошлой неделе!.
Я ходила в музей. = Я была в музее. - I have been to the museum.
Я ездила в Милан. = Я была в Милане. - I have been to Milan.
These constructions have the same meaning in the Past tense.
Please notice that:
Ходила (куда?) в музей (the Accusative case)
Была (где?) в музее (the Prepositional case)