What does "flat image" mean? Like all plug-ins, the 3D is an intra-layer rendering. There is no genuine 3D space in AE. That aside you might wanan start with the basics of 3D - create a camera layer and a light that Element can use to calculate perspective and shading.
Make sure the solid layer you have applied Element to is NOT switched to 3D.
I had the same issue. From one of the Custom Views everything worked fine, in my case. If so, this might be the solution you're looking for.
Somehow when I create a Camera it is set to auto orientation OFF.
I haven't figured out why yet but that makes it look flat, although really it isn't. You just have to move up real close and rotate etc.
Select your Camera Layer, click 'reset' under the transform options and you get your 'point of interest' back.
Or go to Layer - Transform - Auto-Orient and choose 'Orient Towards Point of Interest'
I hope this helps any future solution seekers.