It is one file, with zipped content. After downloading the file you should change last part of its name, change it's extension from .zip to .xpi (use "show extensions option", "do not hide extensions", or similar, in your file manager). Without this step, without renaming the file, it will not be seen by Firefox as an add-on. If you have problem with this, I can send you already renamed file via e-mail.
Then click it and "open with..." in "Explorer/Konqueror/Your FileManager" window (sorry, never used Mac), to open it with Firefox.