%Order.OrderId% should work unless you are using a very old version. If you look at Order.cs you should see the following.
public virtual void AddTokens(SiteConfigurationSection section, NameValueCollection tokens) {
//Todo: use reflection to auto populate tokens
tokens.Add("Order.OrderId", OrderId.ToString());
tokens.Add("Order.OrderNumber", OrderNumber);
tokens.Add("Order.Email", Email);
tokens.Add("Order.PaymentMethod", CheckoutPaymentType.Type);
tokens.Add("Order.CreatedDate", Created.ToString());
tokens.Add("Order.TotalAmount", OrderTotalAmount.ToString("C"));
DotShoppingCart Staff
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