Why does Platform Trim not work on Platforms?

@TylarTyger Hello!  It might be helpful if you could take some screenshots of your build so we have a better idea of what's happening. 

In the meantime, when you select the platform trim you want, hover over the floor of the platform and click there to add it; just as if you were placing carpet or tile.  As for the stairs, I've found they can be really tricky.  Make sure you have a full brick of space at the top and bottom of the stairs for sims to enter / exit.  Also, the stairs landing can't be off the lot.  Hope that helps but if it doesn't, let us know.