Yes, julianelizondo is right, you need to make sure your transformer converts 220 to 110v, not the other way around. If you bought the transformer here, then it likely does just that, but double check and make sure. For larger devices you need to also check the wattage, but small electronics like a baby monitor consume almost nothing, so if you do need to buy a transformer, the smallest one will be sufficient.
As a side note, I've noticed that higher end power supplies, such as for a laptop or many cellphone rechargers, can handle 110 or switch automatically to 220. (they will be marked something like input:110/220). In this case I was always buying and carting around (and losing) the little tiny adapters that go from the V shaped or pin-shaped plugs here to the parallel plug we use in the US. What I discovered is that you can take a pair of pliers and twist the US-style plug into a V-shape, and it generally works fine. It's a bit counterintuitive- I would have thought that it would fall out or be difficult to push in all the way, but it actually mates up surprisingly well. IMPORTANT: Do NOT do this if your device is only marked for 110 input. It will likely be fried instantly.