They may be weary or cautious of the crate at first. Let them smell it and wander around it until they are comfortable and at ease with the crate. Try placing several of their favorite chew toys or a favorite blanket in the crate and it will entice them to try it out.
Don't scare your pug puppy. If they start to enter the crate, don't freak them out by pushing them in. This only gives your pug puppy a reason not to trust you.
Once they are in the crate, use a command that your puppy will associate with being in the crate. Repeat the command every time they enter the crate. Always praise your pug puppy for his achievement.
The next obstacle to overcome is actually closing the door to the crate. Your pug puppy must get used to being locked in the crate. Choose another day to practice this step. With puppy watching, close the door. Puppy will only be closed in for a minute or two. Open the door and let puppy come and go if he wishes. When he reenters the crate, repeat the command and give praise. Then, close the door again for a bit longer this time. Try to get up to half an hour.
Your puppy will whine while they are in the crate the first several times. Don't freak out. If you open the door and let him out, you will be rewarding their bad behavior and they will think that whining will work to get them out of their crate every time. Instead, simply tell them to be quiet. If they still whining, try the command again until he hushes up.
You are not being cruel to your pug puppy, but teaching them. Puppies are cute and will wrap you around their little finger but you should be wary. This may seem harmless enough, but an unruly adult dog is not cute at all. They could be big trouble.
Do not use the crate as a babysitter. I advise no more than 6 hours in the crate at any one time. Your pug puppy could develop separation anxiety, abandonment or fear issues if constantly left alone.
Crate training will also allow your pug puppy to travel outside your home with no troubles. They will be more comfortable in the crate so riding in cars or going to unfamiliar places for the first time will be less traumatic for them.