The Primary Reference "Grasshopper" Could Not be Resolved

Warning 1 The primary reference "Grasshopper" could not be resolved because it has an indirect dependency on the framework assembly "System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v2.0". To resolve this problem, either remove the reference "Grasshopper" or retarget your application to a framework version which contains "System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089". 

I am writing some custom components for Grasshopper, and while I have added my references through the Project -> Add Reference and choosing Grasshopper.dll, when compiling, I get a number of errors, and that is, mainly, "using Grasshopper.Kernel;" would be blue underlined: http://puu.sh/5o4XV/e0f9905fbb.png, and my warnings and errors would tell me that "the type or namespace name 'Grasshopper' could not be found".

How do I fix this issue? I don't believe it is anything in my code that is causing the issue. I apologize for this stupid question - I'm very new to C#.