Set up the inflow using a simple hydrograph in the GDS. Save the project and Close GDS. Open the inflow.dat file in NotePad. Replace the place holder hydrograph with your Excel hydrograph. Make sure the first character in the line is H. You can add that to the Excel columns. Make sure to replace all of the tabs with spaces. You can do this by highlight and copy the tab. Then find and replace the tab with 3 or 4 spaces.
Reload the project in GDS and check the hydrograph data. Just making sure it loaded correctly. If it has any tabs, it will look weird in the GDS table.