What you like to do ... there is a working SSH / SSL already on your Mac. If you like to build stunnel you need Xcode with the CLI tools ... and some time. I haven´t tried it but a quick look in the build instructions looks as if there are no big problems.
As promised, here are the steps to build stunnel from source. I used a Lion machine but it is the same on a Mountain Lion machine. There is no need for Fink, MacPorts or Homebrew ... brings more troubles then it helps.
Go to https://www.stunnel.org/downloads.html and download stunnel-5.23.tar.gz and stunnel-5.23.tar.gz.sha256. Open a terminal Utilities => Terminal>. Terminal is case sensitive, before you do something think twice before you press return - there is no undo or redo in terminal. Each line is one line in terminal and needs to press return at the end of the line.
openssl dgst -sha256 stunnel-5.23.tar.gz
Compare the output of the last 2 lines - if it matches all is fine, if not you had a security problem during download.
tar -xzvf stunnel-5.23.tar.gz
./configure && make && make check && sudo make install
The configure script uses autoconf, so put your fingers away from other options because if all is there and supported it will build a fine 64 bit app for you. make check is optionally. sudo make install requires your admin password. During this step you had to input some basic informations (self explaining). The && is used to run one command after each other ONLY if the last was successful.
After all is done your stunned app was installed in /usr/local/bin. To run it, open terminal and type
stunnel3 (see the docs for the difference) and press return.
For the config file type in terminal.
sudo cp stunnel.conf-sample stunnel.conf
sudo pico stunnel.conf
For the documentation of pico use in terminal.
If I had some mistypes of filenames or anything else here use in terminal the following command to list a directory
For the rest, have fun with stunned and the documentation of it !