As of this writing, Dreamcatcher runs as a plugin for Maya, but a newer, browser based version is already in the works, so instead of going through the details of how to run the alpha version of the software, I'll cover the basic concepts of working with Dreamcatcher.
Designing a bike stem is usually done through intuition and stress testing. With Dreamcatcher, you can set the forces that you want the piece to withstand and have the genetic algorithm go through thousands of iterations to arrive at a solution.
The first thing to do is to set your ports.
Ports are the shapes that you want DC to generate a form between. In the case of the bike stem, we want a steerer tube clamp and a handlebar clamp. Those are the ports.
Model these in your 3D modeling program of choice. I use Fusion 360.
Export all your ports as a single STL mesh file.