"missing interface" is usually an error related to opening .z3d files that were created by ZModeler2. In most of cases, these errors are insignificant and indicates a failure to read some ZModeler2-specific data saved in .z3d file. This could be specific material extensions, splines or, possible, lights.
if you get a screwed scene/viewports after loading such a file, it's recommended to use File\Merge command instead, so you can explicitly specify a set of objects you need to get loaded.
If you get this error on attempt to open ZModeler3 .z3d file, then your current copy of ZModeler is missing some plugins that were available and running when the file was saved. Could indicate corrupt installation or plugins version mismatch in current installation.