IPSec isn't that easy than OpenVPN. It's far more complex and it is originally designed for professional use to setup one to one connections between two IPSec endpoints. It wasn't designed for road warriors like OpenVPN and OpenVPN is much better than IPSec for road warrior usage. So IPSec has problems passing NAT without the right options and so on. As you use Linux you have to do a little more than on windows, burt it is possible to get it to work with PSK or with certificates, as you want to.
You have to install strongswan or openswan to use IPSec on Linux, i would recommend using strongswan. Strongswan itself has several configuration files and places to put certificates or PSK keys. To understand all of this you have to read a little strongswan Documentation:
Btw is IPSec on Win the same may be or may not be thing as it is on OSX and to save work and nerves i would also recommend using OpenVPN on Win or OSX...
On Linux you have the biggest chance to get it right, as you can use strongswan, what many linux IPSec servers use also. On OSX the raccoon client is also a little piece of...
you are very kind but I don't want you waste your time. I'm not a skilled user and so I could need basic step by step instructions.
Therefore don't worry, I continue to use openvpn on Ubuntu and IPSec on Windows.