To enable SVN via Proxy on Mac do the following:

1. Open Terminal
2. Type vim ~/.subversion/servers
3. Enter Insert mode by typing and deleting any letter
4. Go to the line [global] at the bottom
5. Change configuration to: 
   
http-proxy-host = 172.28.111.1(your proxy address)
http-proxy-port = 3128 (your proxy port)

6. Don’t forget to remove # symbols, they mean commentary.
7. Press escape
8. Type :wq, which means that you want to exit text editor and save changes
9. Ready! Don’t forget to use proper url for svn which supports WebDAV, etc.

References:

1. http://ru.wikipedia.org/wiki/WebDAV
2. http://www.rubyrobot.org/tutorial/subversion-with-mac-os-x
3. http://www.eng.hawaii.edu/Tutor/vi.html