The library depends on additional jar files. You should download them from this link: www.b4x.com/b4j/files/jPoi_AdditionalJars.zip
Copy the jar files to the additional libraries folder.
I tried that code, it has a few problems
a, can only handle xls workbooks.
b, prints the output doesn't write a csv file.
c, the csv output isn't valid when loaded back into a spreadsheet.
Dim wb As PoiWorkbook
wb.InitializeExisting("...", "", "")
Dim sheet1 As PoiSheet = wb.GetSheet(0)
Dim table As List
table.Initialize
For Each row As PoiRow In sheet1.Rows
Dim cells As List = row.Cells
Dim r(cells.Size) As String
For i = 0 To cells.Size - 1
Dim cell As PoiCell = cells.Get(i)
r(i) = cell.Value
Next
table.Add(r)
Next