3D modellers and computer games makers often have Y as the 'up' axis. Now you need to check the orientation of the model's coordinate space. Next, move the entire model so that the origin sits at 0,0,0 in model space (unless, of course, the modeller was savy enough to set the origin to a real-world location. Without changing the geometry, move the origin to a point relative to the geometry to a point in the model where you know the real-world coordinates. When a model is first made, this point is often roughly somewhere in the middle of the model, just because that is how the artist made it, but that's often not helpful.
This origin is the point that the GIS will use (and the modeling packages) as the reference of the model. In all modelling packages I have used, you can set the location of origin of the model. If you started from a surveyor's plan, that should be easy. You now need to make all the geometry in the model relative to a known point in the real world. I don't think it has full 3D model support yet, but I did read something along the lines that it is intended. This will also restrict your choice of SRID, unless the increasing 3D-functionality in PostGIS can cope - I haven't tried because I always work in meters and avoid the problem altogether. I suppose you could use decimal degrees but that is alien to most 3d modelling packages I've come across and it will send the modeller insane trying to convert from an architect or surveyor's drawing. This will either mean your model is 1000 times too small, or, if they did work correctly in mm, your GIS will probably assume the data are in meters, thus making your model 1000 times too big. I think 3D Max defaults to millimetres and most artists ignore this and just use a 1 unit = 1 meter approach without re-setting first (from first-hand experience!). Having a 1:1 relationship is a good thing because it makes life a lot more simple (i.e. This will probably relate well to meters and 3DS can be set to meters (I believe - I use Blender - which certainly can and so does SketchUp). You need to make sure that you understand the scale and units which the model was built in. I will assume you have access to the model or some modelling package. What you do really depends a lot on how the model has been built and whether you can get access to it. Seeing as nobody gave you an answer, here is a generic approach. I don't use ESRI City Engine, but I do a lot of work integrating GIS into 3d or vice versa depending on your perspective.
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