It is not difficult. First remember where you have saved downloaded soundpool then open Music Maker.
Example your downloaded soundpool is Rockpop vol1
Click on Add styles button(Music Maker) browse and locate folder titled Rockpop vol1 and click OK. It will be added in the soundpool library of Music Maker alongwith instruments.
Important: when you save soundpool please make sure you will not change its location because if you do so, saved project file could not load next time and will show error.
I have made screen shot for you please refer that.
Note: my system is in Japanese you might see some Japanese character sorry.