Is it right to ask for a PhD position at a conference? I'll be at a conference next month to present my paper.
I have a Master's degree in Computer Science and now I'm looking for a PhD with scholarship.
Well, is it right, with a common sense and in a polite way, to ask a professor at conference about a PhD position?
It is definitely OK. You should be more like @JeffE suggests "How do I apply?" rather than "Would you accept me for a PhD?". 

Conference is a great opportunity to meet and discuss with your potential supervisor. You should try to judge, as much as possible, how easy would it be to work with that person for quite a few years. On the other hand, note that high profile academics are used to get approached by random people and you shouldn't be discouraged to formally apply even if they seem way too busy to have a long chat with you.