There are two ways to import data files: with the "Layer: Import" command, or by using drag-and-drop to drag files into the VTBuilder window.
With the Layer: Import command, you will be asked for the kind of data to expect: Elevation, Imagery, and so forth. This is important because many file formats can be interpreted in more than one way. For example, a Shapefile (.shp) can be used to represent Water, Road, or Building features. Since Shapefiles lack any kind of standards for attributes, a computer can't guess what sort of data it really is, unless it knows what to expect. For another example, TIF files can be interpreted as either Imagery or Elevation, so telling VTBuilder makes it clear how to handle your TIF file.
Depending on the layer type, VTBuilder may give you additional dialogs to clarify how to import the data. See the documentation for each layer type (e.g. Structure Layers) for an explanation of the issues involved in importing data of that type. If the data lacks a CRS, then you may be shown the Projection Dialog to specify it.
The fastest and easiest way to import files is to drag-and-drop them from your operating system file manager. Multiple files can be imported at once by dragging over a group of icons. This is really useful when you have a whole directory full of small files which need to be imported and merged.
When you drop files, VTBuilder won't ask you the layer type for each file, but instead will do its best to guess. Un-guessable GIS files like .shp will be imported as Raw layers.
If you drag-and-drop a compressed (archive) file, VTBuilder will quietly unarchive it to a temporary folder, and look in that folder for something to import. If the archive contains a single file, it will act as if you dropped in that file by itself. If the archive contains multiple files, it will look for distinctive patterns like "catd.ddf" and ".dem" to try figure out what to import.
The feature is very convenient for the user, especially for awkward formats like SDTS.
If you encounter a situation where a file is not importing as expected, please let us know.