I think that I know what you are looking for. You want the 3D Modeling workspace that is available in the ACAD menu but you are probably working in the Architecture (ACA) menu. If that is the case, then it is pretty simple to get the 3D Modeling workspace into the Architecture menu using the CUI editor.
Type CUI at the command line.
Select the Transfer Tab at the top of the CUI editor
In the right side of the dialog, select the Open Customization File button and navigate to the folder where the CUIX files are located. On Windows 7 for AutoCAD MEP 2013, they can be found here:
C:\Users\<username>\AppData\Roaming\Autodesk\MEP 2013\enu\Support
Select the acad.CUIX file and select the Open button.
Select 3D Modeling in the Workspaces section and drag it to the left and drop it in the Workspaces section.
Select Apply and Ok
You should now have two workspaces – Architecture and 3D Modeling
I also created a short video with the steps that you can view here
http://screencast.com/t/Sj54gzMm5e0
I hope that this information solves your problem.

David Pothier