Open your chosen Excel sheet.
Press Alt+F11
On the left under Project - VBAProject, right click on VBAProject (Book1.xlsx or whatever your chosen sheet is named)
Click Insert -> Module
This will create Module1
Copy and paste the code there.
Place your cursor somewhere in the DescripeFunction Text
Press the green Run button at the top (Looks like a play button)
You may now use close this window.
To access the formula type =MANNING(PipeSize,Slope,K,Depth,Coefficient)
Press Ctrl+A for help or Ctrl+Shift+A to see the arguments in the formula pane.