If you see following error during running installer it means that not all files have been uploaded or that files have been damaged during upload. List of MISSING and CORRUPTED files is shown bellow.You need to upload these files again and click refresh to run integrity check again. Make sure that you are uploading all files in binary mode.
If everything is ok, system will proceed with installation / update.
If you want to skip integrity check, type the installation with parameter p=F, like ..../install/index.php?p=F