Multiline text in Excel cells

You need to set the row height to accomodate two lines of text.

row.setHeightInPoints((2*sheet.getDefaultRowHeightInPoints()));

You need to set the wrap text = true to get the new line.
Try this :
Here wb is the Workbook.

 CellStyle cs = wb.createCellStyle();
 cs.setWrapText(true);
 cell.setCellStyle(cs);