Quote - Acadia is right!
My additiional runtimes are indeed just like
V3 Runtime Geometries
and not like
V3 Runtime Runtime Geometries

Is that the problem?

Yes, that is the problem :)

If you have a look at your main Curious Labs folder you will see that inside it is a folder called "Poser <version#>"  and inside that is a folder called "Runtime" which contains a bunch of sub folders. 

So you need to have a similar set up.   Just open the folder called "V3 Runtime"  and create a new folder in there called "Runtime" and move your geometries, textures and libraries folders into it and you should be all set :)

When you link to the folder inside Poser, link to the  folder called "V3 Runtime". Poser will search it and find its usual "Runtime" folder and all of the files.

The folder that you have your actual "Runtime" in can be named anything you want. It doesn't have to have the word runtime in it.  For example I have an external runtime called "Props", the path for it is:

D: Poser Runtimes Props Runtime Geometries
D: Poser Runtimes Props Runtime Textures
D: Poser Runtimes Props Runtime Libraries

When I linked that runtime to Poser I browsed for the "Props" folder and not the folder called "runtime" that is inside of it.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi