Alright a quick 101 to approaching someone for a hypnosis scene.
1. Be Interesting- I can't stress this one enough, hypnosis is about the mind and changing it. Thus if you don't know someone you don't get half the fun. Going to someone and saying "Plz tiz me" doesn't give them a reason to. Be yourself, let them get to know you. This is true online and at munches. Talk with everyone, don't just talk with people you want to scene with. Get involved as it lets people get to like you for you, also it lets you approach better. It is so much nicer to have someone say "Hey, I heard you talking about playing Fallout are you excited for the new one?" because hey, you just gave me something about you as well as proving you were listening, you are coming at me as a human.
2. Don't be a doormat- So many subs feel they need to be subby all the time. So not true, especially at an informal event. Approaching a Domme with "Yes Miss, no Miss" and stuff like that is off putting. Its also disrespectful because they didn't consent to this power exchange you are forcing on them.
3. Accept rejection.- Not every hypnotist is going to want to hypnotize you. This could be for many reasons, maybe the rapport isn't there or maybe they just have too much going on to be able to commit to another subject. Accept it politely, if it is time related and you are getting on well, feel free to keep talking to them.
4. Everyone is a human.- This is the thing people tend to miss and my biggest pet peeve. Dominants are not like ones in porn, we go to the bathroom, have crappy days and sometimes spend too long on 4chan. Thus accept a dominant as human as opposed to a trance vending machine. Respect their emotions and deal with the fact in public and informal situations they might be a very different person.