To work with IMDB Movie Shortcode WordPress Plugin, you need to get TMDB API key. You can get it by creating account on TMDB
After installing and activating this plugin, you will get the options settings page for this plugin under Settings > Movie Option.
To add the IMDB movie info in your post and pages, click on IMDB Movie button in your editor page. Here you can put the IMDB ID of the movie, or you can also search the movie.
Notice that, you have to provide the correct TMDB API key in the settings page to show the movie details on posts and pages through shortcode. Also, the movie search functionality during adding the movie from editor button use this API key.