1. The installation is completed and everything seems to work except new product files are not uploading and downloadable products won't save to the database and there is not error message.
Check the file_uploads option in php.ini. When set to Off, this prevents all html forms that are used for file uploads (such as the addproduct.php page) from working. Set it to On and restart Apache.
2. Installation is completed, but products will not upload and there is an error message.
Permission denied.... editproduct.php on line 238
or
Permission denied... editproduct.php on line 248
It seems as if permission is denied due to chmod, but it could also be caused by the owner of the directory or files being different than the user. First, verify that the chmod was actually set. Some hosting services won't allow 777 permission settings, but may not give any warning that the chmods have not been completed.
Try the product thumbnail image upload. If it works, then the trouble appears to be unique to the products directory. There is a .htaccess (hidden) file in the products directory. If the image upload works, try removing the .htaccess file or replace it with a blank .htaccess file.
If the product thumbnail image upload doesn't work and the permissions for /products and /prodimg are set to 777, then it is likely that you have an ownership conflict. Did you upload the files by FTP? View the owner of the scripts and of the products directory. If they are different, for instance one is root and the other is username, you will need to chown through SSH or delete the files from the root and upload or create them as the username.