Turnkey Joomla 2.5 can't install extensions

A look in  log.error shows 

[xxxx] [error] [client x.x.x.x] PHP Fatal error:  Call to a member function isAdmin() on a non-object in /var/www/joomla/administrator/components/com_attachments/update.php on line 17, referer: http://x.x.x.x/administrator/index.php?option=com_installer&view=install

If its just a new site I would blow it away and start again.

If not you can try unzipping core files.

What extension are you installing to get that error?

Does most of the files install before it bombs out or is there none of the files after attempting the install?

The plugin was designed for Joomla 1.5, my mistake.

http://joomlacode.org/gf/project/attachments/ <-- Attachments 

Thank you very much! :)

That should not happen unless the XML install file was modified.

You should get a warning saying "it was  not designed for 2.5"