Proxy Trouble Shooting
For those having trouble with syncing Anki or accessing shared plugins or decks, (typically owing to what looks like a firewall or proxy problem), check that:
you have a working internet connection (can you browse http://ichi2.net/anki/)
you have correctly input your Anki online username and password into Anki (Settings->Preferences->Network)
You have entered your proxy details (Settings->Preferences->Network, and enter your details under Proxy)
- RESTART anki and try sync/download once more.
If it fails again - then download this plugin and place it in your plugins folder (Settings->Plugins->Open Plugin Folder). Restart Anki and review the contents of test_proxy.log in the plugins folder.
If the problem specified in test_proxy.log is an AUTHENTICATION problem then you may need to proceed as below. Also, if you know your proxy is a Microsoft ISA proxy server, there is a good chance you'll also need to proceed as below. Technically speaking, Python (and hence Anki) don't support Microsoft's proprietary authentication mechansim (ntlm) for the proxy. The simplest solution is to place an additional proxy between Anki and your ISA proxy.
Installing An Intermediate Proxy
There are several open-source projects providing a solution to this problem. One such project is cntlm.
Download cntlm at http://sourceforge.net/projects/cntlm/files/
Set it up according to the README file included (you can find a sample config file here)
- Determine if you want it to start at boot or start manually every time.
- Take note of the port number you specify in cntlm's config file. You will need that number to direct Anki's traffic through cntlm.
- Starting cntlm
Open Anki. Change Anki's proxy host to localhost, and port to XXXX where XXXX is the port you set cntlm on (Settings->Preferences->Network)
- Restart Anki and try to sync/download.
Assuming you set all of this up correctly, it should work. If not, panic 
Then re-read the documentation for cntlm. Tweak config in cntlm and Anki. Repeat until solved.