What is a good etiquette to correct a person using wrong title for me? I currently have a masters degree in an engineering field and I don't currently have any PhD degrees (I may study for a doctorate degree in future). It happens that when I go to conferences or scientific occasions, even in the emails I receive from people; they use a doctor prefix for my name; but I do not have a PhD degree.

What is a good etiquette to deal with such thing? Should I directly tell them that I do not have a doctorate degree? Or should I neglect such a mistake by them and don't do anything?

I think it is unethical not to correct such thing, because the person may still think that I have a PhD degree; and such wrong usage of title should be corrected some way. Once a person used doctor title for my name in an email and I just added a line in my reply to his email, stating that I have a masters degree.
I would say this depends on context. When contacting academics, it is common to assume they all have a doctorate. A mass email (or mail-merged etc.) is therefore likely to use Dr. even for a PhD student on the list, because checking would take much more work. Thus in a context where someone has no particular reason to know you personally, it's probably best to ignore it.

On the other hand, if it's coming from someone who who might reasonably be expected to remember specific information about you as a person, I would be inclined to point it out. Personally I put things like that as a PS to an email, or similar, to try not to make a big deal out of it.

Edit: Another thought on the ethics side. I would agree that you should not allow people to continue acting on the assumption you have a PhD when that is not true. However, in many cases where this arises, it is not that the person specifically believes you have a PhD, but that they are not really thinking about it, or doesn't really care.