Some of the free programs that come to mind are in the lightweight category in terms of capabilities and efficiencies needed to get the job done. To paraphrase an old saying, “if you want something for nothing, don’t be disappointed if that’s what you get: nothing.”
Lightweight and free apps are along the lines of BeatBuilder (search for it here on the forum) and Aria Maestosa (for Mac and PC).
What you should be looking for is a DAW (digital audio workstation). Keep in mind that you’ll have a potentially steep and frustrating learning curve but once you’ve become familiar with your DAW (and the nuances of programming MIDI for the BeatBuddy (BB)), you’ll be happier. Here’s some sorta free ones: https://thehomerecordings.com/free-daw/
Some are more capable than others for example, Garageband does not export to MIDI, which you’ll need for adding MIDI files to the BB.
If you want to jump up the price chain, you can try Reaper for free and then it’s $69 if you decide to use it. For Macs, Logic Pro X is available to try 90 days for free. Forum users have their own preferences and will freely share their experience and opinions on what works best for them.