View source for Why can't you install any extensions?

=== File Ownership Advice from ianmac ===
At the heart of the issue is file ownership. There are generally two main server users that end up owning your files - the FTP user, and the Apache/PHP user. Obviously, when you upload files using FTP, the FTP user ends up owning them. Therefore, if you give a file 755 permissions, then '''only''' the FTP user can write to that file.

If you install Joomla! without the FTP layer, the files it creates are owned by the Apache/PHP user. If you give the file 755 permissions, '''only''' the Apache/PHP user can write to that file.

Just to emphasize, the fact that these username and passwords happen to be the same has no effect. They are different subsystems and are unrelated. It may be convenient, but will not solve your permission issues.

There are two approaches to take:
# Upload all the files via cPanel. This will generally result in all of the files being owned by the Apache/PHP user. Ensure that all Joomla! files in the root directory are writable, so that the installer can create the ''configuration.php'' file. Then, install Joomla! '''without''' the FTP layer.
# upload all the files using FTP. This will result in all of the files being owned by the FTP user. Make sure that your Joomla! root directory is writable, again, so that the installer can create the ''configuration.php'' file. Then install Joomla! '''with''' the FTP layer.

Ensure that your cache folders are owned by the Apache/PHP user because these files are written by PHP. (Cache because writing using the PHP user is much faster than via FTP.)

If you apply these principles and choose either the first or the second approach, you'll get better results and extension installation should work properly via the Backend interface. Mixing the two approaches will cause you no end of grief.

A good way to check that everything is in order is to browse to the Administrator section of your site and browse to {{rarr|Help,System Info,Directory Permissions}}.

If you don't have the FTP layer enabled, it is important that everything show up as Writable. If you do have the FTP layer enabled, it is important that your two Cache directories show up as writable. It is most likely okay that the rest show up as unwriteable, because Joomla! can write these files using the stored FTP settings.

If you choose to use the FTP layer and you still have trouble, reupload the Joomla! 1.5 package onto your server using your FTP client and the FTP user that you specified in Joomla!'s global configuration. If your FTP client reports permission problems when trying to do this, contact your host for further assistance.