In reply to sbellock:
I did it on the second line as you can see. It gives the error still.
Is it that the modport is used for synthesis purpose only and not for testbench?
//---------Wrapper------
module add_wrap(aif);
adif.dut aif;
add i_add( .a(aif.a), .b(aif.b), .sum(aif.sum), .cout(aif.cout));
endmodule
Thank you once again for your response.