Hi
I followed the instructions in post Plugin Store behind firewall and this is now working for me (find the certificate for proxy, export it, import into keystore).
I got the certificate by opening Certificate Manager (in Windows), doing a search for the name of the proxy provider, then exporting the certicate I found).
I supplied the node address of the proxy under the proxy settings as well + port number (no username/password required).
Regards,
Dan