Workshop can only be accessed from a desktop or laptop computer, it cannot be run on a cellphone or tablet.
How you access Workshop will depend on whether the specific space is setup for viewing with the original version of Showcase or the new Beta version.
If you see a yellow label next to the 3D Showcase section title, you're using the original version. In this configuration the workshop button can be found near the upper right corner of Showcase.
If you don't see the yellow label, you're using the Beta version. In this configuration you begin editing the Space by clicking the pencil icon near the top left corner of 3D Showcase.
Once the Space is loaded, the button to access Workshop will appear in the bottom right corner of 3D Showcase.