Double click on track button opens sound browser. From here you can decide from which place sounds will be available for assigning to track – from project sound pool or from +Drive. To select this place press left cursor button and in menu select View pool/+Drive.
If you want to add sounds to your project pool - go to sound manager and select needed sounds by Yes button (small tick appears near selected sound). Then press Right cursor button and select Copy to menu item – another menu appears. Select sound pool menu item. All your selected sounds will be copied to sound pool and you can lock them to trigs. Here you also have options to save selected sounds in any sound bank of +Drive, so you can organize your banks as you want. The same procedure is for saving sounds from sound pool to +Drive. Just select sound pool for showing in sound manager by pressing Left cursor button and selecting View sound pool menu item.