Incase you are using the Mine-imator Community Build/Mine-imator Community Edition,
please make sure your version is up-to-date since the previous one had a bug wich broke the world importer.
If you are using Vanilla 1.0.6, make sure that the file you want to import is a .schematic file.
If you don't have a schematic file of the piece you want to import, make sure to use the world importer instead.