Edit: This issue is solved with the latest version of the Template Chooser. Please, download the latest version.
With the latest builds of Joomla 1.5, the Template Chooser doesn't change the templates any more. We're trying to get a solution from the joomla core team, but so far without result. The problem is inside the joomla core as the function to change the template doesn't function as it should. It can be solved by changing a line in the file /includes/application.php. In the function getTemplate, you'll find the line:
if ($template = $this->get('setTemplate')) {
Replace this line with following line:
After this, the Template Chooser should be able to change templates again.