Public Overridable Sub PlaylistGetByIndex( _
   ByVal _nIndex As Integer, _
   ByRef _pdblFileOffset As Double, _
   ByRef _pbsPath As String, _
   ByRef _ppPlaylistItem As MItem _
) 

public virtual void PlaylistGetByIndex( 
   int _nIndex,
   out double _pdblFileOffset,
   out string _pbsPath,
   out MItem _ppPlaylistItem
)

public procedure PlaylistGetByIndex( 
    _nIndex: Integer;
   Out  _pdblFileOffset: Double;
   Out  _pbsPath: String;
   Out  _ppPlaylistItem: MItem
); virtual; 

public: virtual void PlaylistGetByIndex( 
   int _nIndex,
   [PARAMFLAG::Out] double _pdblFileOffset,
   [PARAMFLAG::Out] BSTR* _pbsPath,
   [PARAMFLAG::Out] MItem* _ppPlaylistItem
) 

Parameters

_nIndex

playlist item index (use -1 for the current item, -2 for the current break or qued item and -3 for qued item only in case of breaks)

_pdblFileOffset

file offset from the start

_pbsPath

item path

_ppPlaylistItem

playlist item MItem object

Here is example of how to add command to the playlist item:

Dim myIndex As Integer = -1
Dim myItem As MItem
Dim strPath As String
Dim dblPos As Double
myPlaylist.PlaylistGetByIndex(1, dblPos, strPath, myItem)
Dim myTargetItem As MItem
myPlaylist.PlaylistCommandAdd("mute", "true", myItem, myIndex, myTargetItem)

int myIndex = -1;
MItem myItem;
string strPath;
double dblPos;
myPlaylist.PlaylistGetByIndex(1, out dblPos, out strPath, out myItem);
MItem myTargetItem;
myPlaylist.PlaylistCommandAdd("mute", "true", myItem, ref myIndex, out myTargetItem);

var
myIndex: integer;
myItem, myTargetItem: MItem;
strPath: widestring;
dblPos: double;
begin
myPlaylist.PlaylistGetByIndex(1, dblPos, strPath, myItem);
myPlaylist.PlaylistCommandAdd('mute', 'true', myItem, myIndex, myTargetItem);
end;

Reference

MItemClass Class
MItemClass Members