The directory where TileMill stores files for projects and exports. If you want to keep your current projects, move the directory before changing this setting.
If your computer uses a HTTP proxy, you will need to specify the details for TileMill to work correctly. The proxy details need to be entered in the form of
http://user:pass@hostname:port where user, pass, hostname and port are replaced with the relevant proxy details. After entering and saving this setting, you will need to restart TileMill. You should then be able to make use of TileMill’s features requiring network access.
Note: TileMill does not support proxies for authorization. To authorize, open
http://localhost:20009 in a browser after starting TileMill.