As I discuss in this answer, the Sauptika Parva of the Mahabharata describes how after Ashwatthama launched a Brahmastra at the womb of Abhimanu's wife Uttara, Krishna put the following curse on Ashwatthama:
As regards thyself, all wise men know thee for a coward and a sinful wretch! Always engaged in sinful acts, thou art the slayer of children. For this reason, thou must have to bear the fruit of these thy sins. For 3,000 years thou shalt wander over this earth, without a companion and without being able to talk with anyone. Alone and without anybody by thy side, thou shalt wander through diverse countries, O wretch, thou shalt have no place in the midst of men. The stench of pus and blood shall emanate from thee, and inaccessible forests and dreary moors shall be thy abode! Thou shalt wander over the Earth, O thou of sinful soul, with the weight of all diseases on thee.
So at least for the duration of the curse, the answer to the question "Where is Ashwatthama" is "wandering all over the Earth". But there's some reason to believe that the curse is over by now, because Krishna said that it would last 3000 years, and Krishna died in 3102 BC, so assuming Krishna was talking about normal years the curse ended over two millennia ago.
In any case, Ashwatthama made clear what his plans would be after the end of the curse; he told the sage Vyasa "With thyself among all men, O holy one, I shall live! Let the words of this illustrious and foremost of men become true!" So he's presumably living with Vyasa now. And as I discuss in this answer, the time he will have spent with Vyasa will enable him to become the Veda Vyasa (i.e. the compiler of the Vedas) in the next Dwapara Yuga.