You can add .mp3 and .wav files to the library either by dragging one or several files to the library window, clicking the Add... button at the top of the window, or selecting the Add to Library function from the context menu. You can perform certain actions by single- or double-clicking items, or drag them to a different location in the library. You can also access certain item-specific functionality in the context menu (usually configured to pop up when clicking the right mouse button on an item in the library).
Simply dragging a file to a different location or folder will move the file, dragging a file to a different location while holding down the CTRL key will swap the files. Files can be removed from the library either by hitting the DELETE key or selecting Delete from Library from the context menu. Hitting the RETURN key is equivalent with double-clicking the currently selected file.