Import 3D models when the model format does not have a defined coordinate system

1. Use either an existing point feature class or create one through the ArcCatalog window, assigning the desired coordinate system: Creating a new shapefile

2. Begin an edit session on this feature class and digitize a point in the location where the model is to be placed: A quick tour of editing

3. Convert this point(s) to a multipatch feature class. There are a number of ways in which to achieve this, among these methods are the Feature to 3D by Attribute and Layer 3D to Feature Class geoprocessing tools: Creating 3D feature data

4. Add this multipatch to ArcScene and begin a 3D edit session. Select the point created above through use of the edit placement tool.

5. Navigate to the 3D editor dropdown and select Replace with Model. Select a previously created model from the list of supported model types: Using the Replace With Model tool for multipatches

This workflow allows for the use of the 3D editing shortcuts in order to move, scale, and rotate the imported 3D models. This is particularly advantageous when models are imported, as the need for spatial adjustment can be averted.

Alternatively, if a multipatch feature class has been created the 3D model can be placed interactively using the insert tool: Placing 3D models using the Insert tool

Note:
Attention should be given to the complexity of the model being imported.  Highly detailed models with complex geometries may not import into the ArcGIS 3D environment properly. Simplifying these models such as through the CityEngine export dialogue for .dae files may be advisable.