Multisim and DAC (summing op amp)
The 741 can provide about 20mA, but you're asking for much more than that.  Bump all your resistor values by a factor of at least 20, if not 100.  Real circuits are not built with resistors that small.  Also, the outputs of the 741 are not rail to rail, and can only reach within a few volts of the rails.  Change the opportunity amp power supplies to +/- 15

Lastly, voltmeters need to be wired in parallel, not in series.