You can clone with
HTTPS or Subversion.
For users behind proxies, TileMill breaks at multiple places where an internet connection is needed. It can hang at startup (due to delayed fetching from npm repos), when adding remote layers, when trying to access the sample projects (since initial data cannot be downloaded), and when trying to upload exports to MapBox hosting.
This is an issue to capture all previous proxy related tickets, and to discuss solutions.
Possible solutions include:
@tmcw helpfully mentioned https://banu.com/tinyproxy/ for a potentially useful tool to test whatever fix is made here.
Also, thanks to @tmcw - node-get supports HTTP_PROXY, which is what allowed the workaround mentioned at #1454 (comment)
Upstream pull request at npm (npm proxy gets passed to the RegClient) npm/npm#2549
Current progress is at #1550
closed when #1550 merged (in 883a838)