I'm under the impression that some MiniDisc players have the ability to transfer their files to a computer in digital format. My unit does not have that capability. If yours does, then, depending on the format, you may not have to do any conversion. But if your unit is like most of them, you'll have to convert the files from their analog output to digital files. This is easily done on the computer using an analog connection from the MD player to the computer, and any program, such as Audacity (free download) that can do the analog to digital conversion. Choose the file format that you ultimately want on your B2: FLAC, or MP3.
Once you have all the songs converted to individual track files, make sure they are grouped into folders for Artist \ Album \ tracks. You will need a USB stick or disk that is formatted as FAT32 (no other format will work, not exFAT, not NTFS, nothing else). If it's not already in FAT32 format, the B2 can do that for you.
Then copy the music files to the USB disk under a folder named music. Once that is all setup, use the Import USB C (or A) command to copy all the files onto the B2.
Since I don't know your level of understanding, I did not go into too much detail at any one point. So if you need clarification on anything, post back here and someone will be glad to give you more details.