How to enable 3D?
There is no enable button. To create a 3D DGN file you should: - open a 3D file- or create a 3D file by using a 3D seed file, - or you can export your 2D file to 3D by going to File > Export > 3D
http://communities.bentley.com/products/microstation/w/microstation__wiki/13810.3d-feature-modeling-microstation
http://communities.bentley.com/communities/other_communities/askinga/w/askinga/3d-microstation