Forum Url: http://lostworldex.smfforfree3.com
Here is the code

<script type='text/javascript'>
/*
Guest Redirect
Made by: Sarah
http://slyvion.co.nr
*/

if(document.getElementById('userlinksguest').innerHTML.match(/welcome guest/i) != null && window.location != ('http://Forum.smfforfree3.com/index.php?action=register') && window.location != ('http://Forum.smfforfree3.com/index.php?action=login')){
window.location.replace("http://Forum.smfforfree3.com/index.php?action=register")
}
</script>

Replace the bold areas.
To your forum name.

Just give your Guest access to see the boards ..   in each Boards settings checkmark Guests.

Then in the Guest permissions limit what they can and cannot do.. dont allow them to post.

If they try to they will know they cant unless they register.. it will be clear to them.

I just realized the problem.   You are using the old version of the simplemachines software.  (1.0)     A long time ago there was the ability to set the software so that Guests could not see the forum at all.   You must have had it set that way.   SMF disabled that ability but you now do not have the ability to change it as the setting no longer exists in your admin panel.

I did not have your forum url when you first posed this question so I was not aware of this.

Solution:   If you want to continue to use 1.0 and do not wish to have your forums software upgraded to the newer version then there is no solution.   You will always have that notice when not logged into your forum.    We are aware of it, therefore, it is not against the TOS.

Solution 2:  If you would like to have your forum upgraded there is a link inside your Admin Panel, near the top for upgrading the forum.   I can also send a note to the  Administrator to let him know you wish to do so.    The new version has more features, is more secure, more options to choose from, more stylesheets to choose from.    You may want to give that some thought.    (the new version looks like the support forum)   Also I would say that 95% of the Stylesheets in the Stylesheet index were created for the new version.

You will not lose any of the threads or posts, will have all your membergroups, settings , members,etc.
Go ahead and use the upgrade link and Ill also  send a note to him for you.

I do not know how soon he will do it.  You'll see a screen telling you that the upgrade is in process (if i remember correctly) when it is happening though.

You are welcome Georgie.  I sent on the message to him.   When it is done do make sure (although you should have no problems) to check through all of the settings.  Poke around everywhere and you'll see new toys to play with     If you have any questions check back in here and ask us.   There are many guides in the Guides and Tutorial Section  (index is at the top of it) for the new version so that should help you out 

oh heck, all I did was pass the message on to SMF.. he did the dirty work ...   you are more then welcome though.