Simply download a brush package from any of numerous websites such as myPhotoshopBrushes.com, QBrushes, BrushKing, etc, then unzip it if necessary, to get to the .abr file.
Open the Brushes pallet by double-clicking any of the brush tools in the Pixelmator toolbox or from the menu choose View - Show Brushes. Click the little Gear icon, ⚙, in the bottom right corner of the pallet.