"missing a directive or reference"

I'm very new at C# and coding. I get this error constantly "the type or namespace could not be found. Are you missing a directive or reference"
Here's the code. TrtReview is the namespace. I'm getting the 'missing directive error' on ConfigurationProperty.  The code is enfolded in a class library. But this code actually works on another page with another 
using System; using System.Web; using System.Configuration; using System.Web.Configuration; using TrtReview; public class TheTrtReviewSection : ConfigurationSection { [ConfigurationProperty("defaultConnectionStringName", DefaultValue = "LocalSqlServer")] public string DefaultConnectionStringName { get { return (string)this["defaultConnectionStringName"]; } set { this["DefaultConnectionStringName"] = value; } }
I fixed it by putting a reference from th ...