Here's what you should do before you even think of approaching the restaurant. Simply go and eat there, don't mention that you do magic or anything like that. Just go to the restaurant, eat and observe the staff and the place. Then when you are done, leave and be on your way.
This will help you decide if you want to work at the place and also will give you a sense of the vibe of the restaurant and also tell you if they are a dead place or a busy popular place. (You don't want to work for a dead place.).
Next, I'd suggest you call the place and get some basic information about the place. Is it Corporate owned, as well as simply complementing the General Manager and boosting his/her ego. Once hat is done, send a letter to the place telling them you had a good time etc etc and don't mention magic or anything like that. Wait about a week or so and then show up during the afternoon and talk to the GM in person and do your pitch.