The current version of Raspbian is 8, aka. jessie. Raspbian is Debian compiled for the Pi,1 with some configuration tweaks and an appropriately customized kernel.
The current version of Debian is also 8, and a major difference between it and previous versions (Debian has been around for several decades) is that the init system, formerly a spin on the venerable SysV init (which is older than Debian itself) was replaced with systemd. GNU/Linux, like other unix derived operating systems such as Mac OS X, relies on an init system of one form or another. Systemd is a fairly new one first adopted by Fedora a number of years ago, and since then it has been gradually picked up by other distros (winning favor over Canonical/Ubuntu's upstart).
The problem is that there doesn't appear to be any such directory
Inittab wasn't a directory, it was a file. Part of what it did was determine the default runlevel, which corresponds to the "X" in
/etc/rcX.d. Systemd does not use runlevels, the equivalent would be targets (primarily, "graphical" or plain "multi-user").
Fortunately, Debian has left in some backward compatible functionality and the
rcX.d directories are still there and the stuff in them is still run. Running something from them was never really as simple as just placing a script there -- possibly you are paraphrasing -- but in any case, you can use them exactly the same way as before (if "they" say it worked, it still should). The runlevel you want to use is probably 2, or possibly 3. However, there is no harm in doing whatever it is you are supposed to do with the script for levels 2, 3, 4, and 5. Only one of them is actually used, so it is common practice to target those 4 (hence if you look, they are almost identical).
Another possibility, as dastaan points out, is to simply invoke the script from
rc.local, which also still works. Make sure you use the full path and fork it by placing an
& after that.
BTW, I don't think systemd is any harder to learn than SysV, and although there is much griping from people having to learn new things "when the old one was fine", if you haven't learned much of either one yet, you might as well start with systemd and ignore SysV, because SysV is pretty much dead, at least in the GNU/Linux world.
1. Hence the first version of Raspbian was 7, not 1.