What shows up in the Workbench Parameter Set is a mixture of Geometric Parameters that come from DesignModeler (or SpaceClaim) and Model Parameters that come from Mechanical. Don't bother trying to create a Parameter in DesignModeler and expect to use it in Mechanical. It doesn't work that way.
In Mechanical, if you see a box, you can click that box and a P will appear and cause that value to show up in the Parameter Set. There is such a box next to the Preload field in Bolt Pretension.