8051 Move Data To/From External Memor
movx a,  @dptr copies 8 bit data byte from external memory location addressed by 16 bit number stored in dptr (obviously,  if it is properly interfaced). 

For e.g. if you want to copy byte (i.e. 8 bit)  data stored at 1500h in your external memory.  You'd write :

mov dptr,  #1500h

movx a,  @dptr

Also, keep in mind that dptr (data pointer register)  is actuality combination of two 8-bit special function registers,  dph and dpl (having addresses 83h and 82h,  in internal RAM  respectively).

You might want to re-read atleast all indirect addressing mode instructions.