My very brief browse of the instruction you link to suggest that these instructions are for a standalone install. As the TKL MediaWiki appliance is already configured to host the wiki you'll need to alter the Apache config. And in my somewhat limited Apache knowledge this is how you'd do it (but be aware that there may be other/better ways).

[edit] This doesn't work as requested by the OP. rather than creating a sub webdir of the url, it makes the DaloRadius page the webroot. See post below for proper solution.

You'll want to set up a new Apache site/virtual host. So to create the new file:

nano /etc/apache2/sites-available/daloradius

and then paste/write something like this:

NameVirtualHost *:80 #Remove this to make https only
NameVirtualHost *:443 #Remove this to make http only

#Remove this to make https only
<VirtualHost *:80>
        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/daloradius/
</VirtualHost>

#Remove this to make http only
<VirtualHost *:443>
        SSLEngine on
        SSLCertificateFile /etc/ssl/certs/cert.pem
        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/daloradius/
</VirtualHost>

ScriptAlias /cgi-bin/ /var/www/cgi-bin/

<Directory /var/www/daloradius/>
        Options Indexes FollowSymLinks MultiViews
        Order allow,deny
        allow from all
</Directory>

If you only want it available via https then remove the 'NameVirtualHost *:80' line and the <VirtualHost *:80> to </VirtualHost> section (remove the <VirtualHost *:443></VirtualHost> if you want it only available, or will only work via http). Also you shouldn't actually need the VirtualHost declarations at the start (ie NameVirtualHost *:...) as they should already be set within the existing site, I just included them to be sure... Exit and save (<Ctrl><x>).

Then you will need to enable your new site and restart Apache:

a2ensite daloradius
service apache2 restart

Assuming that I haven't lead you astray, you should now have the DaloRadius WebUI showing when you navigate to http://<ipaddress>/daloradius

Let me know how you go...

Scratch my first post...!

Instead try this. Delete the DaloRadius site we made earlier and make a new one.

rm /etc/apache2/sites-available/daloradius
nano /etc/apache2/sites-available/daloradius

This time put this in it:

Alias /daloradius/ /var/www/daloradius/

<Directory /var/www/daloradius/>
        Options Indexes FollowSymLinks MultiViews
        DirectoryIndex index.html
        Order allow,deny
        allow from all
</Directory>

You should now have your desired behaviour, although you will need to tweak it a little. Where it says index.html you may need to change that to the index file of your DaloRadius index page (eg index.php/index.htm or whatever it is). Also when I was testing to make sure it would work as intended I noticed that http://<domain-or-ip>/daloradius/ works, http://<domain-or-ip>/daloradius (without the last slash) doesn't. To work around this I just created a symlink in the MediaWiki doc root pointing to the DaloRadius folder like this:

ln -s /var/www/daloradius/ /var/lib/mediwiki/daloradius

(double check the MediaWiki path as I don't have acess to the MW server now - although I did test this when I did). Now http://<domain-or-ip>/daloradius nicely redirects to http://<domain-or-ip>/daloradius/

If you disabled the DaloRadius site (to get your wiki back) then re-enable it (using a2ensite - otherwise skip that). Obviously you'll also need to restart apache, but you can also just reload the settings (which is a little quicker) with:

service apache2 reload

As an afterthough, it occurred to me that perhaps you could just change the alias declaration (in etc/apache2/sites-available/daloradius) to 'Alias /daloradius ...' (rather than /daloradius/ - ie no trailing slash) but I didn't test it so I don't know if it'd work.

Have a crack at that and see if that works for you, be great to hear how you go.