The IDbCommand and IDbConnection ... MySql.Data could not be found エラーについて

NHibernate.HibernateException: The IDbCommand and IDbConnection implementation in the assembly MySql.Data could not be found. Ensure that the assembly MySql.Data is located in the application directory or in the Global Assembly Cache. If the assembly is in the GAC, use <qualifyAssembly/> element in the application configuration file to specify the full name of the assembly.

[HibernateException: The IDbCommand and IDbConnection implementation in the assembly MySql.Data could not be found. Ensure that the assembly MySql.Data is located in the application directory or in the Global Assembly Cache. If the assembly is in the GAC, use <qualifyAssembly/> element in the application configuration file to specify the full name of the assembly.]
   NHibernate.Driver.ReflectionBasedDriver..ctor(String driverAssemblyName, String connectionTypeName, String commandTypeName) +225
   NHibernate.Driver.MySqlDataDriver..ctor() +47

[TargetInvocationException: 呼び出しのターゲットが例外をスローしました。]
   System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandle& ctor, Boolean& bNeedSecurityCheck) +0
   System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean fillCache) +86
   System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache) +230
   System.Activator.CreateInstance(Type type, Boolean nonPublic) +67
   System.Activator.CreateInstance(Type type) +6
   NHibernate.Connection.ConnectionProvider.ConfigureDriver(IDictionary`2 settings) +172

[HibernateException: Could not create the driver from NHibernate.Driver.MySqlDataDriver.]
   NHibernate.Connection.ConnectionProvider.ConfigureDriver(IDictionary`2 settings) +319
   NHibernate.Connection.ConnectionProvider.Configure(IDictionary`2 settings) +207
   NHibernate.Connection.ConnectionProviderFactory.NewConnectionProvider(IDictionary`2 settings) +490
   NHibernate.Cfg.SettingsFactory.BuildSettings(IDictionary`2 properties) +1293
   NHibernate.Cfg.Configuration.BuildSettings() +48
   NHibernate.Cfg.Configuration.BuildSessionFactory() +206