The main solution does not compile from Git, it is missing the file:
Source file 'SS\UserModel\IndexedColors.cs' could not be found
Comments: ** Comment from web user: BruceCarson **
Source file 'SS\UserModel\IndexedColors.cs' could not be found
Comments: ** Comment from web user: BruceCarson **
The one that's linked to from your home page (https://npoi.codeplex.com/) :
https://github.com/tonyqus/npoi