First, open the Avocode App Preferences (CMD/CTRL + ,). Stay in the General tab and in the last input field, fill in your proxy server address.
If your proxy requires authentication, you will need to put your username and password into the url like this: "http://username:email@example.com:3128"
Guide for Plugins (environment variables)
Avocode’s Sketch & Photoshop plugins respect proxy server configurations set by HTTP_PROXY and HTTPS_PROXY environment variables. To use your proxy, you will need to set both of these variables to your proxy server address.
In the guide below, replace "http://yourproxy.com:3128" with the address to your proxy server.
The values of both HTTP_PROXY and HTTPS_PROXY must be the same.
If your proxy requires authentication, you will need to enter your username and password into both HTTP(S)_PROXY variables before the domain like this: "http://username:firstname.lastname@example.org:3128"
On Windows 8 press the Windows key and type "environment," select Edit environment variables for your account:
On Windows 7 you can find this in the Control panel, see this document
Create New user variable HTTP_PROXY, value will be your proxy url.
Create a second variable HTTPS_PROXY - the value should be the same
Confirm your changes by pressing OK
These changes will work permanently until you delete these variables.
You will need to run these commands in Terminal to set both env variables:
launchctl setenv HTTP_PROXY http://yourproxy.com:3128
launchctl setenv HTTPS_PROXY http://yourproxy.com:3128
To make the change work permanently (even after reboot), add both lines to /etc/launchd.conf - this doesn't work on Yosemite though, you would need to create a custom Apple script
You can run this command before launching Avocode in Terminal:
To make the change permanent, you will need to change your ~/.profile (for bash) or other files depending on your terminal. See this stack exchange answer for more info.