I have had a bit of a nightmare getting a freshly installed Joomla 1.5 system to accept uploaded extensions. These are the steps I took:
- disable safe mode for the domain in the plesk panel domain setup
- Set permissions to 777 for:
/components /tmp /administrator/components
- disable FTP layer in Joomola global config
This should allow basic uploading of components, although depending upon the component you may have to set 777 permissions for some other folders too.