Created Unassigned: npoi.xssf.usermodel [11874]

Hi all,

I wanna use npoi to manipulate .xlsx files in VS2012/2010. To do so I should import NPOI.XSSF.UserModel, but when I add the npoi.dll and try to import that, there is no XSSF type of NPOI at Using part. I mean that there is no "Using NPOI.XSSF"
Any help?