A rat pup is about the same size (maybe a few grams larger) than an adult mouse. A snake that has taken 5 meals should have no problem with an adult mouse.

Just leave the F/T mouse in the cage overnight. Toss it in as late as you can at night. I usually wait until past 12am to feed my snakes.

Don't start panicking just yet. You have only tried to feed her once. She is probably still a little bit scared from the journey to your place and is probably still adjusting to her new home. I would wait 5 days before offering her food again. Then she should be pretty hungry!