Nope. You haven't missed anything. In order to switch to a new runtime you need to go to the top of the runtime hierarchy until the Poser folder shows a red dot, and then  double click a runtime folder listed there.

That is the reason some people don't like multiple external runtimes and opt to just have a bunch of sub folders inside their main poser runtime in order to organize.

I however have 40 runtimes and have 5 or 6 always in poser and the others I add and remove as I need them. I don't mind switching runtimes; it doesn't take long, and it is by far the best way for someone like me who is anal about organization to sort my files.  LOL

"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