WordPress MU

HI I'm a newbie here and not sure if this is where to post this.I am also having problems, with sub domains.
my site is http://www.uk2uonline.com .I want to add users to my domain. ie, username.uk2uonline.com.
I set up a test user mixle the email I received stated
To activate your blog, please click the following link:

http://mixle.uk2uonline.com/wp-activate.php?key=fd2aa2af24076a0c

After you activate, you will receive *another email* with your login.

After you activate, you can visit your blog here:

http://mixle.uk2uonline.com/
But when I click on the link to activat I get
Firefox can't find the server at mixle.uk2uonline.com.
I think this is an .htaccess fault, but don't know how to change it. my .htaccess is as follows
RewriteEngine On
RewriteBase /

#uploaded files
RewriteRule ^(.*/)?files/$ index.php [L]
RewriteRule ^(.*/)?files/(.*) wp-content/blogs.php?file=$2 [L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule . - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-.*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
.
Can someone please help it would be very much appreciated. Thanks uk2uonline

You added the right DNS A record, and edited apache's config file to set-up subdomains correctly?

Thanks for reply, how do I add the right DNS A record (where do I find it)and how do I edit apache's config file to set-up subdomains correctly? ( where do I find that)
As I said I am new to all this learning it slowly.
Thanks

http://www.blogopreneur.com/2006/11/06/installing-wordpress-mu-on-a-cpanelwhm-server/