In order to open .fxb/.fxp preset files in the plugin browser, the files must be copied to the following folder:

C:\Program Files\VSTplugins\Waldorf\Largo Sounds

Macintosh HD\Library\Audio\Presets\Waldorf\Largo

Also, you can use the Add Scan Folder option to add another folderfor saving and loading .fxb/.fxp.files. For this, you need to right-click Local Library in the plugin browser.