Have you seen the FAQ entry What if I’m behind a proxy??
… edit your “servers” configuration file to indicate which proxy to use. The files location depends on your operating system. On Linux or Unix it is located in the directory “~/.subversion”. On Windows it is in “%APPDATA%\Subversion”. (Try “echo %APPDATA%”, note this is a hidden directory.)
For me this involved uncommenting and setting the following lines:
#http-proxy-host=my.proxy
#http-proxy-port=80
#http-proxy-username=[username]
#http-proxy-password=[password]
On command line : nano ~/.subversion/servers