The error on that page indicates that it can't find a folder and file that it needs to run to continue. It thinks that 'WA_SecurityAssist/Helper_PHP.php' is in a folder one level higher up than it actually is (i.e. - in your site's root directory)
As it seeme that your PowerCMS files and folders are in the correct places (i.e. - all in the 'CMS' directory) then check that the settings in your CMS/WA_Globals/WA_Globals.php file are correct.
They lines you should check are the 'localRoot' and 'remoteRoot' lines which for you should look like this:
$WAGLOBAL_Site_Name = "CMS Admin" ;
$WAGLOBAL_localRoot = "/CMS/ "; //relative path to your PowerCMS files on your local testing server.
$WAGLOBAL_remoteRoot = "/CMS/ "; //relative path to your PowerCMS files on your remote server.
// Admin Settings
$WAGLOBAL_Admin_UserName = "yuoradminusername" ;
$WAGLOBAL_Admin_Password = "youradminpassword" ;
Check that these are correct on the CMS/WA_Globals/WA_Globals.php file that has been uploaded to your site.
Let us know how you get on.