You unlock a city bike through the app. In the app you'll get an overview of where the stations are, and how many bikes and locks that are available on each of them. You can download the app for both iPhone and Android.
In order to unlock a bike you need to stand next to a station:
See to that the station that shows up in the app is the same one that you're standing next to (if another station shows up, check out this guide)
Press Unlock bike
The app will show you what lock your bike is stationed in
Go to the bike and press the button on the handlebars if prompted. It may take up to 40 seconds after you press the button to unlock.
Take the bike out of the lock