There are three software components that you will need: an Ada compiler, the PLplot library, and the Ada bindings.
You will need an Ada compiler in order to use the Ada PLplot bindings. There are several compilers available. Here, we will focus on the free, open source compiler that is included with the GNU Compiler Collection, (gcc) which is at the center of much of the open source software movement. The gcc Ada compiler is known as GNAT, for GNU NYU Ada Translator, where NYU stands for New York University. (Although GNAT was originally developed at NYU, it has for many years been developed and supported commercially by AdaCore with academic and pro versions available.)
Your computer may already have GNAT installed, or you can download it from gcc.gnu.org. Another route to obtaining GNAT is from the AdaCore page, libre2.adacore.com. There are versions for many operating systems and processors including Apple's OS X or its open source version Darwin, Linux, and Windows. The gcc and AdaCore versions differ in their licenses. Download the version that you need and follow the installation instructions.
PLplot can be downloaded from the PLplot project page at sourceforge.net. Follow the installation instructions after downloading. The installation process requires that your computer has CMake installed. OS X users can try installing PLplot in its entirety from MacPorts but that activity is not officially supported by the PLplot developers. The advantage of using MacPorts is that all installation dependencies are automatically installed for you.
The third major software component is the bindings themselves. Since they are currently included with the PLplot software itself, there is no need to download them from another place.
The bindings themselves are six Ada source files named (using GNAT filename extensions) plplot.ads, plplot.adb, plplot_traditional.ads, plplot_traditional.adb, plplothin.ads, and plplotthin.adb. There are two additional files, plplot_auxiliary.ads and plplot_auxililary.adb which will be discussed later, in Section 9. These can be stored somewhere on your system's search paths for easy access.