If you are using Magix Music Maker, You will see on the Left side Soundpool/Styles then Instruments. You will have to drag the item of your choice and drop on the desired place in arranger.( arranger is an empty place with lines.
I think if you refer page 6 of my pdf book it will show you with photos how to do and what to even you will find some interesting stuff too. Do not worry about the title of the book it will be useful for Magix Music Maker other version.