MC tries to guess if an imported video file is a movie, home video, or a TV show based on a number of factors including the duration of the video and the filename. When it guesses, it sets the [Media Sub Type] tag appropriately. You can easily change this though, if it guesses wrong, by simply changing that [Media Sub Type] tag.
Select the file and open the Tag Action Window. If the [Media Sub Type] field isn't showing, click the little button in the upper-left corner of the Tag Action Window and choose Also Show > Media Sub Type. Then, change it from TV Show to Movie for that file.
That movie is probably fairly short (less than an hour and a half), which MC considers to be a TV Show in most cases.
When you do the metadata lookup on a single file, you can search for anything in the box you'd like. However, you want to set the Media Sub Type appropriately first. If you search for a TV Show, it looks at theTVDB and not at the various movie sources (TMDB, Rotten Tomatoes, Wikipedia). So, that's likely why it isn't finding any results, because it is looking in theTVDB for a tv show, not TMDB. Once you fix the Media Sub Type, it will probably work, but if not (because the name is not quite right or whatever), you can manually change the search in the box to whatever you'd like to find the proper result, then select it.