How to find the current?
Put a '+' where the current enters a resistor and a '-' where it exits. Then start at Vo and go on a journey through the components doing what the signs tell you.

So for the circuit on the left, starting from Vo you go down by V and that puts you at (Vo-V) at the top of R, then down by iR to get to ground (= 0). Hence, Vo-V-iR =0.

For the circuit on the right, start at Vo, then go down by iR to get to V, then down by V to get to 0. Hence, Vo-iR-V =0.

You can go around any path or circuit like this, it's a really handy trick.

E.g. alternatively, start at ground. For the left circuit: from 0, up by iR then up by V to get to Vo, or 0+iR+V=Vo. 

For the circuit on right: 0+V+iR =Vo