Step by step :
1. Install GUI for OpenVPN :
apt-get install network-manager-openvpn
2. Restart network manager
3. Open the Network Manager GUI, select the VPN tab and then the ‘Add’ button. Select OpenVPN as the VPN type in the opening requester and press ‘Create’. In the next window add the OpenVPN’s server name as the ‘Gateway’, set ‘Type’ to ‘Certificates (TLS)’, point ‘User Certificate’ to your user certificate, ‘CA Certificate’ to your CA certificate and ‘Private Key’ to your private key file. Use the advanced button to enable compression or other special settings you set on the server.
Noted; file format .P12 can used to all certificates.
4. Now try to establish your VPN.