Some TTG plugins that reside on the web server require write access to selected folders in order to perform correctly. Examples are the directory for saving orders for the Cart, and the gallery upload directory for Publisher.
Access is granted by modifying permissions of the folder via your FTP software. Generally, this is performed by connecting to your server, browsing to the folder, and right-clicking the folder to access properties. The level of permissions required depends on the hosting environment. Some hosts require read/write/execute access for the role of user but only read and execute access for the roles of group and world (known here as granting '755' access). Other hosts require read/write/execute access for the roles of user, group and world (known here as granting '777' access). Some hosts that require 755 permissions are Bluehost, Lunarpages and Web Hosting Hub. It is advised to try 755 access first, and then if unsuccessful in writing to the directory, to then try 777 access.
As an example, setting 777 permissions for the Publisher's 'galleries' folder looks like this in Transmit:
A similar dialogue is available in Filezilla by right-clicking the folder and selecting 'File Permissions':