Problem using scope with linq

I must specify the isolation level  for some transaction, so I've tried something like in the code snippet.

But we always have an exception.

I'm sorry, I've VS in french and all exception message are in french too :(

Information about the exception:

   à System.Data.EntityClient.EntityConnection.OpenStoreConnectionIf(Boolean openCondition, DbConnection storeConnectionToOpen, DbConnection originalConnection, String exceptionCode, String attemptedOperation, Boolean& closeStoreConnectionOnFailure)
   à System.Data.EntityClient.EntityConnection.Open()
   à System.Data.Objects.ObjectContext.EnsureConnection()
   à System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   à System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   à System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   à System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__0[TResult](IEnumerable`1 sequence)
   à System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
   à System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
   à System.Linq.Queryable.First[TSource](IQueryable`1 source)
   à pages_Default.makeLoan(geststockEntities gse) dans d:\WorkSpaces\geststock\GestStock2\pages\makeLoan.aspx.cs:ligne 168
   à QueryHelper.insertDB(InsertDbDelegate methodThatWriteDB, IsolationLevel isoLevel, Page senderPage) dans xxxxx\QueryHelper.cs:ligne 47


ex = {"Échec du fournisseur sous-jacent sur Open."}
//If I translate this, it would be something like : "Failure of the underlying provider on Open."

InnerException = {"L'accès réseau de MSDTC (Distributed Transaction Manager) a été désactivé. Activez DTC pour l'accès réseau dans la configuration de sécurité de MSDTC à l'aide de l'outil administratif des services de composants."}
//Translation
Access network MSDTC (Distributed Transaction Manager) has been disabled. Activate DTC for network access in the security configuration for MSDTC using the Services administrative tool components.


I've a MS SQL 2005 standard Database and I currently only use the "debug" and "local" server of visual studio.

So what can I make?

Is there a better way to manage isolation level for a transaction?

try
        {
            using (var scope = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions() { IsolationLevel = isoLevel}))
            {
                using (geststockEntities gse = new geststockEntities(cs_admin))
                {
                    methodThatWriteDB(gse);
                    objectsAdded = gse.SaveChanges();
                    if (objectsAdded == 0) throw new Exception();
                    scope.Complete();
                    addUserMessage(senderPage, "Insertion OK");
                } 
            }
        }
        catch (Exception ex)
        {
            addErrorMessage(senderPage, "Error message");
        }

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