The 18d is a separator, it decouples from both sides where as a the 18-A decopuler only decouples from one side which is why it has an arrow pointing to the side that decouples. Same for the tr/tv.
Edit:
Find answers to questions like this here.
First have you tried both 64bit and 32bit versions of KSP? Also where did you specifically try putting CSteamworks.dll? I notice it's in the main KSP directory and ksp_x64_data\plugins on mine which may mean if you are using 32bit (the default for steam) this might be part of the problem. Though you'd think if it's in the same directory the program runs from it would find it. Also mine is dated 4/19/2016 so make sure your copy is not any older.
