To set up the VPN via the gui network-manager, you need to provide a key and some certificates.
They are actually all in the client.ovpn file that you can download via a web browser connection to your access server. It is then a matter of copying them out of that text file and pasting them into separate files.
But there is an easier way because the installation of the server includes a script which does that for you.
Instructions are here at the time of writing:
This is what I did:
I have openvpn_as installed on Ubuntu Server 14.04.
The scripts directory is therefore
as root or sudo run (on the server)
./sacli -a 'openvpn' -o OUTPUT_DIRECTORY --cn 'your user' get5
The first user, 'openvpn', needs to be an admin user. I didn't change the default when I set up my server, so I provided "openvpn" to this command.
'your user' is one of the users you have set up.
I used /tmp as the output directory since I'm the only user on this machine.
What I did was then copy those five files into a directory back on my client machine (which also happens to be Ubuntu, this time 14.10). I called it "openvpn_config".
You will have five files: a client.ovpn file, and four certificate-type files.
Now repeat the process of importing a client.ovpn file to a new VPN connection via the gui network manager. You can now assign the secret key (the client.key file) , ca.crt and client.crt files on the main dialog.
You still need to find a home for the ta.key file.
For this, you need to go into the advanced settings folder and set the TLS key file as well (to ta.key). Make the direction setting "1".
provide the user and password, and save. You may need to log out and log in again. Then try your connection.
messages are written to syslog to
sudo tail -f /var/log/syslog
will show you what's happening as the connection is attempted. You will notice permission warnings.