By a cell index from the specified range of cells, or by row and column offsets relative to the cell range. Use the CellRange.Item property. See the How to: Access a Range of Cells document to learn how to access an individual range of cells in a worksheet.
Dim workbook As IWorkbook = spreadsheetControl1.Document
Dim worksheet As Worksheet = workbook.Worksheets(0)
Dim cellA1 As Cell = worksheet(0, 0) ' Cell A1
Dim cellB2 As Cell = worksheet.Cells("B2") ' Cell B2
Dim cellC2 As Cell = worksheet.Cells(1, 2) ' Cell C2
Dim cellD3 As Cell = worksheet.Cells(2, 3) ' Cell D3
Dim cellE4 As Cell = worksheet.Rows(3)("E") ' Cell E4
Dim cellF4 As Cell = worksheet.Rows("4")(5) ' Cell F4
Dim cellG5 As Cell = worksheet.Columns(6)(4) ' Cell G5
Dim cellH6 As Cell = worksheet.Columns("H")(5) ' Cell H6
Dim cellI7 As Cell = worksheet.Columns("I")("7") ' Cell I7
' Access a cell from the range of cells.
Dim cellB5 As Cell = worksheet.Range("B3:D6")(6) ' Cell B5
Dim cellD6 As Cell = worksheet.Range("B3:D6")(3, 2) ' Cell D6
The following images demonstrate how rows and columns are indexed in a worksheet and in a cell range.