Installing GNUPlot is straightforward. You may be able to find a "binary" precompiled version for the Intel Mac on the Internet, but these often lag well behind the releases (and behind Windows binaries). There are "disk image" (.dmg) files that are easy to add to the Applications directory, but these are not what's needed for SYN. An installation from sources (downloaded from the GNUPlot Internet homepage) is well documented, and automatically produces a Mac-tailored version. But the only easy path to obtaining the necessary GNU gcc compiler, etc., is through the Apple "Xcode" package, which comes with a new Mac. (It's on the MAC OS X installation DVD -- not the Applications DVD -- and can be installed -- the whole of Xcode, not just gcc -- by inserting the DVD -- not by restarting.) You can also get Xcode from Apple, but not without registration and a long download.
It is unfashionable to execute programs directly out of a directory by typing their names. If there is a program named "doit" in the current directory, it must be run with the command "./doit" rather than the more natural "doit". But the SYN tools are best used with the latter convention. To set it up, the "PATH" environment variable must be changed. In a "terminal" window the command is:
export PATH=$PATH:.
To install this feature for all future logins, add that command to the file ".profile" in your home directory. If you are already using ".profile" or other shell-startup files or already managing your PATH, you'll know what to do. (There are, of course, more complicated, sophisticated ways to arrange that typing its name will execute a program. If you are uncomfortable with that convention, you could establish a special user account used only for SYN, since each user has a unique ".profile".)
SYN tools are used in a "terminal" command-line window that provides access to FreeBSD directly with a "bash" shell. For users without UNIX command skills, a mixture of GUI Mac operations to manipulate files and "terminal" to execute SYN tools on those files is indicated. For example, you might want to do all your file unpacking and copying in a Mac window by clicking and dragging, etc., then switch to the directory you created in a "terminal" window. Files can be directly edited in a "terminal" window with (say) "vi". If a file already exists, it can be edited with TextEdit, but beware of creating files there. TextEdit will add a spurious extension and may modify the file's format.
In non-Intel Macs prior to the conversion to FreeBSD, things would be far more difficult. To port SYN to those earlier Macs is a project that is probably too horrible to contemplate. Both Perl and GNUPlot are flaky, and the use of ":" as a path separator is a mess.