There are 2 types of validators: field validators and entity validators. Field validators are generated, entity validators aren't, you have to implement these yourself by implementing IEntityValidator.
You're passing in the generated validator? What does "overload resolution failed error" mean exactly? (compile error I'm sure, that it can't find a proper method to use, because type not found). If you're passing in an IEntityValidator implementation, use:
entity.EntityValidatorToUse = new ItemValidator()