to the program:

      ORG    0
Loop: MOV    C,P3.0
      MOV    P3.1,C
      SJMP   Loop

You will have to set the XBR0 SFR (bit 0) to reflect the change of P0.4 and P0.5

     ORG     0
     MOV     XBR0,#1
...

You might have to define the SFR Location as well if the compiler does not recognize it.

     EQU     XBR0,#0E1h

Unfortunately, I'm not at all familiar with this chip, so there may be other things that have to be done.

Are you using 11.059MHz crystal?

Jon