To install:
Unzip the brush pack somewhere on your hard drive. Brushes are unziped .sut files. In Manga studio in the upper left corner of Sub Tool palette there is a button that opens a drop down list. In this list there is Import sub tool option. Click it and navigate to any .sut file. Alternatively, you can just drag and drop any .sut file (or many at once) from your file browser to empty space in Sub tool palette.