Thanks for all the info! I did it slightly different.
You can also download all files (incl. subfolders and the files in the subfolders) from https://wpcom-themes.svn.automattic.com/reddle/ to your PC and then log-in with FTP to your webserver, where you host your website and wordpress. (no need to create a zip file first)
Then create the folder named reddle on your webserver in:
wp-content/themes/
Then upload all files and subfolders into the following folder:
wp-content/themes/reddle
Then go to themes in wordpress and you will find reddle added as a new theme.
