In the sequence table there is a column named 'Dilution' - here you input the dilution factor before acquiring the data file. The screenshot below is taken from ChemStation C.01.07 SR2
If you are reprocessing previously acquired data, you need to edit the sequence.....
and enter the dilution factor in the 'Dilution' column and then click 'Reprocess' from within the sequence table.
Hope this helps. Please come back if you need more information.