In order to access your Xoops installation via the URL you should do the following changes:

1. You should change the following line in your Xoops mainfile.php. Here is the path to do this change:

        define('XOOPS_URL', 'http://serveraddress/~username/path_to_Xoops_installation/');

You can edit it through cPanel --> File Manager --> find the file --> click on it's name and on the right side click 'Edit'. Or you can download it to your computer and edit it locally, then upload it again and replace the old file with the new.