Not sure if this is too late but if you binded a key for the mod, the menu pops up for playing music and stuff. Then you should see 5 buttons in a row on the top like the play/pause button. The 5th one should be like a skip button or something. Above that it's labeled "presets". When you press that 5th button it should take you to the next part where you can assign your songs. You can choose whether its for a boss or a biome by pressing that same button or the first button from the far left to go back. Make sure you have a song in the menu first though, whatever song is highlighted in the menu that's the song that is going to be assigned to whatever you want. Hope this helps! If not I'm sorry C;.

Ah I almost forgot, some other mods WILL override this mod. I was using infernum's music mod and my assigned songs were showing up but not playing. Infernum's ost was playing instead so don't forget to remove your other music mods.