From the press release:
Kobo, a global eReading service, today announced the Kobo Desktop Application, a free, downloadable application now available from www.kobobooks.com that enables users to read, build a digital library, and shop for eBooks directly from their computer or laptop. The new application also allows Kobo customers with 3rd party eReaders, such as the Sony Reader, to easily connect to Kobo to download or purchase content.
The new Kobo Desktop Application offers a personalized, secure and central destination where users can manage their entire Kobo reading experience. Through the Desktop Application customers can instantly access their library and download their purchases directly to their computer and read offline without an Internet connection. Additionally, the Kobo Desktop Application enables users to shop for eBooks through the integrated Kobo eBook Store and download content to their desktop to easily build and manage their library
You can find the application here. It supports both Windows and Mac.