I'm not sure what you're aiming for here. If you want to become a conference organiser, it is possible to do this as a graduate without further study. The skills you will need to develop are in sales, marketing, networking, time management and self-organisation, and strong market awareness (e.g. relevant to the audience of the conferences you are organising).
If you want to do some additional study though, then the above skills may be useful as a guide to what to look for. Schools with a strong collegial spirit and extracurricular activities will give you more opportunity to practise networking. You may wish to look at courses with a marketing focus, but since you don't already have a business background, you would probably be just as well to go for a general business qualification to build up your commercial awareness. Go for an MSc/MA rather than an MBA though as this will suit your experience level better.
Since conference organising is more a sales activity, it would be helpful to get the best possible name on your CV as you can as this will help you establish your credibility with clients and help you to stand out from the crowd further down the line. You can consult league tables from publications such as the FT, Economist, Businessweek etc. for more guidance on business schools' standings.
Of course, if you are simply after an "international" career then probably the best way to choose a business school is by its reputation/ranking (the affects the likelihood people will have heard of it in other countries) and the international composition of its students and faculty. Some league tables actually list the internationalism of programmes as a separate ranking criterion, so keep a look out for this.