Hi metnovas,
put this code in your button event (note: C#)
IXMLDOMNode nodePicture = thisXDocument.DOM.selectSingleNode("my:myFields/my:PicturePath");
//create new OpenFileDialog object
OpenFileDialog ofdlg = new OpenFileDialog();
ofdlg.Title = "File Browser" ; //sets the file dialog box title
ofdlg.InitialDirectory = @"c:\" ; //sets the initial directory displayed by the file dialog box
ofdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*" ; //sets the current file name filter string
ofdlg.FilterIndex = 2 ; //sets the index of the filter currently selected in the file dialog box
ofdlg.RestoreDirectory = true ; //restore current directory before closing
if(ofdlg.ShowDialog() == DialogResult.OK)
{
//write FileName/Path into the textbox
nodePicture.text = ofdlg.FileName ;
}