Get free OpenVPN Connect app from Appstore.
Choose server you want to connect
then download keys for the selected country of connection from - locations
Open the configuration file from your mail by clicking on "Open using OpenVPN" link
Click ADD, indicate your login and password form the registration mail. Check Save password to avoid typing it every time you need to connect and click ADD.
Allow system to save configuration of VPN.
Activate the VPN-connection by moving the slider to the right.
Now you are using VPN!
If you don't want to use OpenVPN Connect for some reason, you may still use VPN connection on your iOS device setting it up using built in L2TP-client as instructed below.