To install the Designer extension for Magento, follow these instructions:
Before uploading any files or making any changes, please make a backup copy of both your database and your files.
In the Magento admin panel, select menu item:
System > Magento Connect > Magento Connect Manager
In the area labeled Install New Extensions, paste the extension key below then hit "Install" button:
http://connect20.magentocommerce.com/community/Print_Science_Designer_Web_to_Print
Delete all the content in the folder:
[magento store]/var/cache