sudo apt-get update
sudo apt-get install gnuplot
will do the trick. (You may have done the first already for getting other software.) The GNUPlot version installed may not be the latest, but that is not a concern. If you want the newest version or can't use the Debian package installer, perhaps the easiest course is to compile from sources; instructions are at the SourceForge GNUPlot homepage.
A "terminal" or "console" window is needed to use the command-line mode of working. The command-line environment is UNIX's forte, and there should be no difficulty with file/directory names and using an editor like "vi" or "emacs".
It is unfashionable to execute programs directly out of a directory by typing their names. In most systems 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, with the command (for the "bash" shell):
To install this feature for all future logins, add that command to the file ".profile" in your home directory (again, for "bash"). 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".)