Two answers to this question.
1. As far as I understand you cannot insert an Autocad 3D component straight into an Inventor assembly. I might be wrong on this, tried it but I get an error message. Granted I work on 2008, might have changed since then.
2. To open an Autocad 3d model in Inventor:
a. Click on open, make sure file type includes *.dwg
b. Select the file you want to open.
c. Next to the "Open" you will find two others - "Find..." and "Options..."
d. Click on "Options..." - it will open a smaller window giving you 2 options to choose from - "Open" or "Import" - by default "Open" is selected.
e. Select "Import" and click on "Ok" - you will return to the "File open" dialogue box.
f. Click on "Open" now.
g. You should now see a dialogue box opening "DWG/DXG File Wizard"
h. First pages should not give you any option but "AutoCAD or AutoCAD Mechanical File" which should be selected by default.
i. Click on "Next"
j. Next page has more options but the following should be selected: "3D Solids" "Detected units" and under "Destination for 2D data" "New drawing"
k. If you get more experience you might start playing with the settings here to get different results.
l. Click on "Finish" and IV should now go through the process of opening the Autocad 3D model. Normally it creates 2 screens, one being an *.idw and the other either an *.ipt or an *.iam. I discard the *.idw because it normally contains nothing, and save the *.ipt or the *.iam as a new file.
However, when starting this process you might get an error message in the beginning stating something about the validity or license of the software (again an issue with 2008). Just uninstall OEMView through your Control Panel and it should work.
This should help you.
Regards
Kobus