How to access connectionString within class

I've created a class to pull data from an SQL database.

SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LocalSqlServer"].ConnectionString);
SqlDataAdapter sqlAdapter = new SqlDataAdapter(statement, conn);
DataSet sqlDataSet = new DataSet();
sqlAdapter.Fill(sqlDataSet);
return sqlDataSet;

however when I run it, the connectionString brings up an error, I'm assuming this is ...