I am not sure what was used to create the file, but I am told it is a 3D model of the different home types that we have on our project and I want to add them to our Civil 3D design items that are currently in InfraWorks.
dgn files are used by Bentley Microstation. You will need to convert the file to dwg. You should be able to open the dgn in Civil3d and then Save As or Export to dwg. You could also do the conversion in Microstation.
I have found the best tool is File->Import->Import from other formats, in C3d.
For some reason, that works much better than opening and saveas.
type MENUBAR to get pulldowns if hidden. And RIBBONCLOSE while at it, much better
The previous reply "I have found the best tool is File->Import->Import from other formats, in C3d" Is correct for what I have found to be the best way to get DGN files into Civil 3D.
Start a new drawing, Xref your Civil 3D design work in, and then import the DGN. You may have to line up the DGN information with your Civil 3D design [so it is located correctly]. If they are 3D objects representing houses, you should be able to save, and import them into Infraworks as an AutoCAD DWG.
If the DGN doesn't come in as 3D models, you can still use the outline of the house. Take the house outlines, and turn them into closed polylines, and create a block from them. You can import the AutoCAD DWG block file, and set the type to buildings, and assign heights. You will also want to use the drape option so the houses sit on your surface. You can find this option on the source tab.