I think you might be migrating from your Web application to your desktop (e.g console application) or using a console application. I think your VS.NET project’s Target framework is “.NET Framework 4 Client Profile”, then you should use the Aspose.Cells.dll component file located in "net3.5_ClientProfile" folder @ your installation directory. If your VS.NET project’s Target framework is “.NET Framework 4” or other (greater than or equal to 2.0), then you may simply use the Aspose.Cells.dll component file located in "net2.0" folder.
For your information, after installing the version, you will get different Dlls complied versions (net1.1, net2.0, net3.5_ClientProfile etc.). Please use "Aspose.Cells.Dll" in "net3.5_ClientProfile" at your installation directory if your target framework for your desktop application is .NET client profile.
Alternatively, you may change the Target Framework for your desktop application to other than .NET client profile, now you do not need to change the existing Aspose.Cells.Dll version.
Let us know if you still find the issue.