I saved a stair (test_stair.gsm) in the ArchiStairsLib folder.
It shows up in the list of files,however, I cannot place it in the floorplan using the normalprocedure for placing Objects.

How do I save stairs,created for one project and used then on another project ?

Dear Conrado,

probably you misunderstood the way in which ArchiStair works: ArchiStair doesn’t save stair objects (like StairMaker does)!
You can save (using the save setting button) the configuration of your stair but not the stair object.
If you want use the same stair in a different project you should copy the stair from the original project and paste it into the new one or (maybe more comfortable) save a module file (including just the stair) that can be imported (also by Drag & Drop) in any other ArchiCAD project.