Need help with a R-2R DAC Circuit
The problem is that your bit inputs must be at either a logical 1 (Vcc) or a logical zero, (GND) as they would be if they were connected to the outputs of, say, a counter like an HC191. 

You've shown them floating when the switch is OFF, which won't work properly if you're using an R-2R ladder.

This works, and here are the LTspice files so you can play with the circuit if you want to.