In the SPH click on the "Actiongroup" tab and click actiongroup 1 (or any other group).
Once you done that select the Rapier engine(s).
then, you will see a bar in the "Actiongroup" tab with all the things the Engine can do.
Click "Switch mode".
You should now be able to use actiongroup 1 for your ssto.
Once in Flight, click "1" on your keyboard and your Engines will "Switch mode"
I hope that will solve your problem.
I also suggest looking at some ssto building video's on Youtube.
PixelatedPig's post in How do I switch rocket modes with action groups? was marked as the answer