Select "Add Folder to Favorites..." and navigate to the folder where you unzipped your new presets.
Unzip the preset pack you downloaded.
Serum 2 (and Serum 1, as the process is identical). how to install serum 2 presets
In Serum’s browser (left side), click the "File" menu at the very top of the browser panel. Select "Add Folder to Favorites