To date, the apparitions alleged to have occurred in Medjugorje have not been approved. Indeed, two successive bishops of the diocese in which Medjugorje is located and the former Yugoslavian bishops conference have gone on record to state that nothing supernatural is occurring at Medjugorje. As ordinary authority for the validity of apparitions belongs to the diocesan bishop, such statements should be treated seriously by Catholics.
The Church does not prevent anyone from going wherever they want to pray -- including the sites where alleged apparitions have taken place -- but, in the case of Medjugorje, ecclesial authorities have asked that parishes not sponsor pilgrimages to Medjugorje because such sponsorship implies an approval for the alleged apparitions that has not been given to them by the Church.
Recommended reading:
Medjugorje by Jimmy Akin
Medjugorje: Secrets, Messages, Vocations, Prayers, Confessions, Commissions
by Bishop Ratko Peric (Diocese of Mostar-Duvno)