You should set 777 permissions for:
3 directories:
public_html/admin
public_html/admin/includes/languages/english
public_html/includes/boxes
10 files:
public_html/admin/includes/boxes/tools.php
public_html/admin/includes/database_tables.php
public_html/admin/includes/filenames.php
public_html/admin/includes/languages/english.php
public_html/contact_us.php
public_html/includes/column_left.php
public_html/includes/database_tables.php
public_html/includes/filenames.php
public_html/includes/languages/english.php
public_html/index.php
Then refresh the page.
NOTE: Don't forget to change the permissions back at the end of the installation. (755 for the directories and 644 for the files)