You got an ac adapter meant for a different part of the globe as wbogacz stated, but if you still have the adapter from the Nexus or well any device just about you should be able to use it with no problems. You may be able to look in the packaging and many ac adapters sent these days can be converted (usually the plug face slides away and you can grab another plug face (meant for that charger not another) and slide the US plug face on to it if they shipped it with your device and it just has a different plug adapter connected to it.
Possible Immediate Solution
I use my Nexus 7 tablet ac adapters to charge my Battery backup pack, my phone, my ps4 controller and a good number of other USB devices. If you look at the label of an adapter you have as long as it says it does an Output of no more than 2 amps than the vast majority of devices will be OK with using it (some newer devices have fast charge and their chargers may read up to 5v while devices and the charger should be able to handle that if you want to be safer I do not use USB AC Adapters on devices that do not fast charge (when plugged in the lock screen will read charging, charging rapidly, or charging slowly (rare)) that have outputs above 2A.
Easiest solution (though slower charge)
If totally unsure you can always connect the USB cable to a computer USB port and that will almost never cause any issues (though it will only slowly charge most devices).
- I can plug my fast charging phone in to a 2amp charger and it will still read charging quickly this does not mean the charger is unsafe for use with other devices just that the device is still capable of fast charging when connected to it.
How to be absolutely sure a charger is safe for a bettery
- You can also be detailed and look at your physical battery (or documentation if it is sealed and you can not see the actual battery) and there will be a label mentioning input Volts if the charger label OUTPUT voltage is equal to or less than the stated INPUT voltage on the battery label you are definitely SAFE to use that charger with that device