You will find it in top left of the page next to poster under Discussions button with Bubble icon, so feel free leave IMDb which abandoned users and plan to delete their content.
You can return back to movie page by clicking on name of movie. Discussions have two categories - General, for general topics regarding movie, Content issues is self explanatory regarding content on main page.
Link from mobile view is currently NOT available, should be rolled out today accoding admin (Travis Bell) words. As temporary solution you can add "discuss" in the end of mobile URL.
I am not in any way affiliated with TMDb and registered there like 2 days ago when finding sad news that IMDb is going to delete my comments I wrote for 14 years and what's worse won't let me discuss movies with other movie fans.
After doing research of alternatives (Rotten Tomatoes, MetaCritic, Letterboxd, Trakt.tv and other) I consider TMDb the best alternative to move on with your ratings, watchlists and discussions, none of other sites mentioned earlier have proper discussions, only other alternativ are threaded comments at Trakt.tv.