Goto your cPanel -> phpMyAdmin. Once in hpMyAdmin, select the PHP-Nuke database from the drop-down menu at left. The page will refresh and the database's tables will be displayed on it. Open the SQL tab (look at the top navigation bar).

In the text field write the following SQL query:

UPDATE `nuke_config` SET `nukeurl` = "full_server_url_to_nuke" WHERE `nuke_config`.`nukeurl` = "current_url";

"full_server_url_to_nuke" - this is the full server URL to your PHP-Nuke installation. For example it could be http://siteground185.com/~username. If the installation is placed in a subfolder, the URL should be http://siteground185.com/~username/subfolder.

"current_url" - the URL the PHP-Nuke is initially setup with.

Once you are ready, click on the GO button to submit the query. If everything goes fine without errors, you should be able to access the PHP-Nuke with the server URL.