Place some articles of clothing of yours in the crate with him/her. Something that has your scent on it like a pillow case, shirt, jacket, blanket.
Also keep the door to the crate open when he/she isn't in it. At nap time, place the puppy in the crate and leave the door open. They need to learn that the crate is a safe place, and not punishment. Maybe sneak a treat in there when they aren't looking and let them find it on their own.
Make sure the pup has a warm soft place to lay inside the crate. I know this is hard, especially when you are using the crate to potty train, but wash the soiled bedding frequently and replace with dry bedding. It can be cold and uncomfortable for him/her laying on just the hard plastic.