Open your web browser and log on to a web-based e-mail service that has a secure web connection by default such as Gmail. (This means, use a web-based service but MAKE SURE https:// not http:// is in the address bar.)
After you are securely logged in, e-mail firstname.lastname@example.org to receive a link to download the client to your computer. Do NOT do this from an e-mail client such as Microsoft Outlook or Thunderbird or the mail client on your mobile phone. Again, use https://.
Follow the instructions and install the program. Psiphon 3 only works on Windows-based systems but the open source project says they are working for versions that work on Mac OS X and Android-based phones.