Audacity is a recording/mixing program - you'll need to either
[A] Record the drum pattern yourself (via something like Garageband, or an electronic kit or drum machine [I just bought a Simmons kit for $199 so that I can do just what you're trying to do] - nowadays, you can get a drum machine pretty cheap on Amazon or the like) , or
[B] Use a pre-recorded drum pattern, and convert the mp3 to an Audacity file (open a new window in Audacity, then drag and drop the mp3 file into it), then once it's an Audacity file, copy/paste/mix it into your music.
Good luck - keep makin' music!
P.S. If I can be of any help, please feel free to email me.
We can beat them just for one day
We can be heroes just for one day