microsoft excel 12.0 object library missing

hi..
I need to export some data which is displayed in datagrid to excel.when i tried to write the code it is not possible because i havent add "microsoft.office.interop.excel" namespace.But when i tried to add it was not possible.Because microsoft excel 12.0 object library is missing.but it is not there to add in the reference..Then how can i make it possible?.Help me please...