Taken from: The WordPress MU Manual

Next you need to set up your sub-domain or sub-directory, database connection details and site email address as follows:

1. Go to your site in your web browser and refresh the page. It should look similar to the screenshot below.

2. In the Blog Addresses section you need to choose whether you would like blogs for the WordPress µ install to use sub-domains or sub-directories. You can not change this later.

a. Sub- domains

If you want to use sub-domains you will need to contact your host and ask them to set up Wildcard dns records for your site e.g. log into your support area of pSek and ask them to set up Wildcard dns records for your site.

b. Sub-directories

This is the easier option because you don’t need to get your hosting company to set up Wildcard dns records for your site.

3. In the Database section you need to enter your database connection details.

These are the same details you set up in your cPanel i.e. you set up a database and username named wmpu and cPanel will have added your domain name to the front of each.

4. In the Site Details section add your email address. Site name can be changed now or later.

5. Once you have entered these entire details click Submit at the bottom of the page.

6. Your site should now look similar to the screenshot below and you should receive an email with your WordPress MU username and password.

7. Click on the log in link to log into the dashboard of your WordPress MU site using the username admin and the password generated when your site was installed.

8. Your blog dashboard will look similar to this:

9. If you click on Visit site link your site will look similar to this:

After this, the final step to complete installation of your WordPress MU is to reset the permissions on the public_html and wp-content directories back to 755.

1. Double click on top folder to close your public_html folder and go up one level.

2. Now right click on your public_html folder and select file permissions.

3. Change numeric value to 755 and then click OK.

4. Double click on your public_html folder to open up the folder and then right click on your wp_content folder and select file permissions.

5. Change numeric value to 755 and then click OK.

Now we’re done with installation we’ll be looking into blog theme options!