This is code taken from my application. The first group shows how I defined the spreadsheet in access, the second shows how to merge cells:
Process_Begin:
'Dimension the application, the workbook, the worksheets
'the column and row titles; the column and row data
Dim objXL As Excel.Application
Dim wbXL As Excel.Workbook
Dim wsXLValues As Excel.Worksheet
Dim wsXLCCS As Excel.Worksheet
'Merge cells in CCS Spreadsheet (sheet2)
wsXLCCS.Range("G1:I1").MergeCells = True
wsXLCCS.Range("J1:L1").MergeCells = True
wsXLCCS.Range("M1:O1").MergeCells = True