I have a master site like master.domain.com, and I have added a slave master.domain.com/slave1 after watching slaveinstall.swf. When I save this new site I get a message saying "you can now map your new domain to the directory /var/www/joomla". What exactly do I have to do?

A catalog /var/www/joomla/multisites/slave1 has been created, and it contains files and links.

But, when I try to enter master.domain.com/slave1 for installing the site I just get "the webpage cannot be found".

What can be wrong in my configuration?

If you have deployed the slave site using the symbolic link, you can ignore the message as it is mainly destinate to person who define a domain or a sub-domain.
In this case, the must assign in their hosting server the name of the domain with the path that is given here.

So in your case, as you have created a slave1 that is a sub-directory, you don't have to setup your hosting server.

When using the master.domain.com/slave1 domain, have you created the directory before deploying the slave site or have you enable the check box to ask create the directory when it is not present.

Check that the directory is created.
Normally, when you have deployed the slave site using Symbolic Links, you should have plenty of links present in the directory.

The directory (/var/www/joomla/multisites/slave1) is created and has plenty of links.

I have tried entering both master.domain.com/multisites/slave1 and master.domain.com/slave1 whithout any luck.

So if you have (/var/www/joomla/multisites/slave1) with plenty of links, this probably means that you have deployed the slave site into this directory.
You can check it when you go in manage site and the you put the cursor over the site id or when you edit the slave site.

If this is correct, this mean that the URL that should work is master.domain.com/multisites/slave1

To check the URL path, I suggest that you use the "hello world" procedure.
In the zip you have 2 files
hello.txt that allow to check the URL path
hello.php that will allow you to check that you have permission to execute script.

Start to put those files into the master and access them with
- master.domain.com/hello.txt
- master.domain.com/hello.php

They should both work.

After that you could put the files into the multisites directory and check that you can execute them again
- master.domain.com/multisites/hello.txt
- master.domain.com/multisites/hello.php
Perhaps you will have only the hello.txt files that will be accessible.
If this is the case, you should check the permission on the multisites directory and the one of hello.php.

If OK, repeat the operation with the multisites/slave1
So copy again the file in this directory and check
- master.domain.com/multisites/hello.txt
- master.domain.com/multisites/hello.php

If the txt is OK and not the PHP, check again the permissions.

The "hello world" worked fine.

I thougt I had tried everything, but now I see that when I access master.domain.com/joomla/multisites/slave1
I get the message "No configuration file found and no installation code available. Exiting..."

And when I access master.domain.com/joomla/multisites/slave1/administrator I get to the login screen for the master site.

You have to start solving the ""No configuration file found and no installation code available. Exiting..."

The reason is that you probably don't have permissoin to access the installation directory.
So continue with the hello.txt and php that you have to copy into the installation directory.

retry to test the permission with the url
- master.domain.com/joomla/multisites/slav...stallation/hello.txt
and
- master.domain.com/joomla/multisites/slav...stallation/hello.php