where to install this driver and how to do it.

Take a look at my roof driver, github.com/Trigger-broom-289/odroid-indi-roof
It's not for an Arduino, however it works from the GPIO of my Odroid-C1 (can be easily changed for the RaspberryPi).

Okay, and thank you.
But what and how to proceed.
What am I supposed to do?
Because I don't know what to do, I have to install the driver on the PC (where there are kstars).
and then install a code on the arduino that the driver commands, right?

In a nutshell, you would need to achieve the following in your case:
(Let's assume for a moment that you run a Raspberry Pi)

Connect the Arduino running your own code to the Raspberry Pi (with the Arduino being the control board to your roof)
On the Raspberry Pi you need to run the indi server and run an Indi driver (your own one)
Your indi driver is the "interface" between indi server and the Arduino


My driver controls the linear actuator, directly from the GPIO pins from my Odroid-C1 (Raspberry Pi like board).

but how do you make and install an Indi pilot ?

Christophe,
before we can advise further, I think you would need to give us more details about your observatory please.
Are you running a Raspberry PI?
How is the roof opening, what system are you using? a motor, a linear actuator etc...

It is extremely difficult for anyone on the forum to provide you with a detailed reply without knowing much about your system.
For example also, do you have to run an Arduino?

My driver was based on Dokeefe's one github.com/dokeeffe/indi-aldiroof
As you can see, people have different systems, it takes time to write a driver, especially for custom observatory.

Hello
So I have a roof with a DC motor, so I have four relays to reverse the current.
If you can translate the CODE, because it's in English, that I made for my arduino you can see the 4 relays and push buttons for manual control and limit switches.

The thing I don't really understand is this driver story, because if I understand (but nobody can confirm it), it has to be installed on the PCLinux(kstars), which is in the observatory, and then kstars has to control the arduino which has the control relays for the roof engine.

I highly recommend that you review (and modify to your wiring) github.com/dokeeffe/indi-aldiroof as he is using an Arduino to control his motor via relays, it's very similar to what you have.
Also, please read the following link indilib.org/about/discover-indi.html where you should be able to understand the all indi concept.

Okay, I've read the link, but how do you do that?
There's still something I don't understand.
It's probably all simple, but I don't understand...

Let us know which operating system you are running please, Mac/Windows/Linux ?

Some links for you
Linux: indilib.org/support/tutorials/169-ekos-o...-complete-guide.html
Windows: indilib.org/support/tutorials/164-using-...s-under-windows.html
Mac: download and install indilib.org/download/osx.html

I'm on linux Ubuntu
I've been using Kstars for 3 years for imaging on my computer which is at the foot of my mount.
To connect to the pc I use "NoMachine" which I find better than the remote version of kstars.

But as I say I don't understand this driver installation story ....

because in "Profile Editor - Kstars" there is a tab "Dome" where is "RollOff simulator", this is where I have to install a new driver ?

Sorry for my ignorance in computer science