I remember the movie from when I was much younger. It started with a line of recently dead souls lining up to enter Hell. They are comparing stories about how they ended up there. The movie ends with one person being sent back to Earth to deliver the plans for a doomsday weapon.
I think the the movie is from the early- to mid-sixties.
I believe this is the black and white film The Devil's Messenger starring Lon Chaney, which was constructed from episodes of a TV anthology series 13 Demon Street (from Sweden, although filmed in English). It's from 1961, which fits with the timeline, and does involve people coming to Hell.
It does indeed start with people lining up to get into Hell. However, the second person to approach the Devil is offered a job, being the Devil's Messenger, to deliver objects that will lead people to commit actions that will get them into Hell more easily. It is these three stories that we see.
The stories are (all quotes are excerpted from reviews on the IMDB page):
Story number one deals with a stressed out photographer in need of a break. He takes a vacation to the mountains, but lets his lustful ways get the better of him as he murders a woman he tries to make advances on. After he returns to the city, the woman suddenly appears in a photo he took. Every time he looks at the picture, the woman appears closer in the picture, as she is making her way towards him to take her revenge.
The next story is about a group of scientists who discover a woman frozen in a block of ice deep in a cave. One scientist becomes obsessed with the woman and will go to any length to free her and make her his own.
The final story concerns a man who has a series of dreams concerning his death. His doctor advises him to go to the building that is in his dreams, in hopes that this will bring him some sort of relief. Instead he finds a gypsy woman who has a vision regarding his death as well.
The ending doesn't seem directly related to any of these stories at all, although the man in the last is one of those given the assignment that you remember from your question:
All three people in these episodes ended up downstairs with a little help from Satanya by planting items for them to help them get there, Mister "D"'s housing complex, somewhat faster. Back to the pit of fire John and Satanya together again are told by Mr. "D" that he has something for them to bring to the surface and give to the world powers to play with: The formula to build a 500 megaton nuclear bomb! With that we see the world explode in a mushroom cloud and with it getting as hot up there as it's down here with Mr. "D". With that Mr. "D" gets to control both upstairs and downstairs of the earth and his pressing housing problems are finally over.
