AutoPLANT is not supported to run on top of AutoCAD Mechanical in a out-of-the-box installation. To run it with AutoCAD Mechanical, you need to configure it to run with a Vanilla AutoCAD profile, while working with AutoPLANT.

Steps to Accomplish

1. Install AutoCAD Mechanical 2012 and AutoPLANT v8i SS4
2. Launch AutoPLANT v8i
3. In Tools->Options->Files remove the following support paths:
   
4. Change the “Main Customization File” to acad.cuix, by default it is located at:
   C:\Users\<<User name>>\appdata\roaming\autodesk\autocad mechanical 2012\r18.2\enu\support
5. The results should look something like this:
   
6. Click OK, Restart AutoPLANT and it will load using Vanilla AutoCAD.
7. Go to Tools->Options->Profiles, export AutoPLANT_PDW811ACAD_A005_409 profile, and import it to other computers if necessary.

See Also

Bentley AutoPLANT