For UI, just use the TestComplete objects as usual. If you need to play with the data in a workbook, use the Excel interop.
' Obtain a reference to the already running Excel, or start a new instance if none is running.
On Error Resume Next
Set objExcel = GetObject( , "Excel.Application")
If Not Err.Number = 0 Then ' If there was an error, there is no existing instance, create one.
Set objExcel = CreateObject("Excel.Application")
End If : Err.Clear : On Error Goto 0
' Get the value of the cell @ Row, Col in the first worksheet of the first workbook of the application instance.
CellValue = objExcel.Workbooks(0).Worksheets(0).Cells(Row, Col).Value