LONGLONGを文字列に変換するには %I64d

LONGLONG llData = 0x0000FFFFFFFFFFFF;
CString strWork;
strWork.Format( "Data = %I64d", llData ); // 10進
strWork.Format( "Data = %I64X", llData ); // 16進

ネタ元

• http://taiyonokumasan.blog47.fc2.com/blog-entry-15.html