I came across the same issue today and just figured out how to do it by browsing the options tool. For those who are using autocad 2010 architecture and are beginners like myself, this is how you activate the 3d workspace.
1) Right click anywhere in the command box near the bottom of the screen
2) Select Options
3) Click on the profiles tab
note: you should see three items
autocad arch (imperial)
autocad arch (metric)
4) Select autocad (not autocad arch imperial or metric)
5) Click "Set Current"
6) Click "ok"
And voila, you should have the option to switch between the default workspaces as aforementioned in the lower right hand corner of the screen.