Les erreurs sur EPPlus

J'ai ajouté EPPlus et l'utilisation de OfficeOpenXml à mon projet.
Cependant, lorsque je lance mon projet, je reçois cette erreur et d'avertissement :

Avertissement :

The referenced assembly "EPPlus" could not be resolved because it has a dependency on "System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which is not in the currently targeted framework ".NETFramework,Version=v4.0,Profile=Client". Please remove references to assemblies not in the targeted framework or consider retargeting your project.

Erreurs :

The type or namespace name 'OfficeOpenXml' could not be found (are you missing a using directive or an assembly reference?)
The type or namespace name 'ExcelPackage' could not be found (are you missing a using directive or an assembly reference?)

Quelqu'un peut-il m'aider à comprendre?

OriginalL'auteur Morilog | 2013-06-03