When you first log in to the web portal you will be prompted to install the desktop software.
Any time after that you can get the software from the web portal at Account Settings > Software Download.
You can also download the software via these links:
Windows Software: http://updates.livedrive.com/updatelivedrive/livedrive-setup.exe
Mac OSX Software: http://updates.livedrive.com/updatelivedrive/livedrivemac.zip