It sounds like you did not copy the install/webassist folder over the webassist folder at the root of the site.
the steps fro updating are to:
1) Copy all of the php files and folders in the Power Store zip file and paste them over the files and folders on your site
2) Copy the install/webassist folder and paste it over the webassist folder at the root of your site
3) run the sql script that matches your update path in the install/scripts folder.