I finally managed to come up with the workflow that Inventor requires for an user to create splines interior and exterior.
As rdyson pointed out in the image attached the spline tool is located inside the Assembly template->in the Design tab.
This is where I was getting stuck. I wanted to simply model the coupler I attached in my first post. Which I did successfully below.
So the tool is in the assembly template because in order to create these splines you need to______???
BINGO!!! HAVE AN ASSEMBLY.
To create the inner splines you need to create a shaft so that the coupler will become the "Hub" when adding the splines. Then when creating the Outer splines, you need to have a hub part. This image might help; the red parts are just dummy parts to obtain the splines I needed for my coupler, hope this helps someone who is having troubles as I did. I found that even though I had to create 2 dummy files, it was still much faster to create these splines for the coupler than actually creating a sketch, and then extruding/revolving a cut.