These types of errors can appear if you install new modules, plugins etc. into the new Joomla Version 1.5. This problem appears most, if Joomla 1.5 was installed automatically by your host. For example the web hoster allinkl. On the one hand the problem is based on the configuration.php which is not correctly set to the file permissions 777 and on the other hand, that there isn´t any FTP User set up.

But you can fix this problem with this instructure:

To change the file permissions of the configuration.php to 777 open your FTP program and right click on the configuration.php. In the Popup menue search for file permissions or CHMOD. If you have found the entry click on it. Now a new window will be open and there you can change the permissions to 777.

If you did this open your Joomla Backend over http://yoursite.com/administrator and search Configuration --> Server --> FTP Settings.

Activate the checkbox for FTP and insert your FTP informations, which leads directly into the main root of your joomla installation.

It is better to create a new FTP account. Finally click Save

Now it should be possible to install modules, plugins etc.