Hi I'm trying to set up my first site.

I have 1 problem after setting up my slave. There is no css on the /installation page.

Site Details:

Site ID: hello.mysite.com
Status: Confirmed
Owner: myuser
List of domain names: hello.mysite.com
Expiration date: <EMPTY>
Master site directory: /var/www/clients/client1/web1/web
Deployed directory: <EMPTY>
Create Directory: <YES>
Create an alias link: <EMPTY> (Tried several attempts)
New Themes Folder: /var/www/clients/client1/web1/web/multisites/hello.mysite.com/templates

DB host name: <NOT_FILLED_YET>
DB: <NOT_FILLED_YET>
DB prefix: <NOT_FILLED_YET>
DB user: <NOT_FILLED_YET>
DB password: <NOT_FILLED_YET>

What am I doing wrong?

I don't understand the question relative to "There is no css on the /installation page"

I see a screenshot of the install 1.5.11 and the installation directory that is restored by JMS is the one distributed by Joomla itself.
I don't know if this is related to the language ?

If you have not removed the "original" installation directory but renamed it into something else, you can try restore your "installation" directory instead of the one restored by JMS.
You could compare the result.

If the CSS is not displayed, I suggest that you have a look into the "page source" and verify the path of the CSS.
Perhaps you could identify the path that is not correct or the file that is missing.

All I can tell you is that Joomla itself has changed many languages things in the installation.
I have posted remarks to joomla core team regarding this kind of issue.
I have also discovered that in the official distribution, they have restore language files of Joomla 1.5.9.
Here it is my bug tracker concerning the language file in the installation
joomlacode.org/gf/project/joomla/tracker...racker_item_id=16600

If you want, you can to send me by email the URL of the slave site that you want to install. I will check the "page source". Perhaps I will find the error in the URLs.

The /installation folder generated by the JMS system, had insufficient rights, I used chmod -R 755 on the folder.

The problem is solved.