Pahy26,
An open day consists of the recruitment team doing a presentation on Emirates and taking in CVs to short-list candidates for their upcoming assessment days.I would recommend doing research on EK as at the end of the presentation they give you a chance to ask questions (try to ask a smart question, as this might make them remember you). To be honest,If i were you i would try to use a conceler on your acne to try to cover it up as much as possible as EK do lay quite a bit of emphasis on apperiance ( your personality is most important, however apperance does play a part,not that you have to look perfect or anything) In regards to the level of English, it needs to be somwhat "fluent".I mean you dont have to be able to read and understand shakespeare or anything but but a high level of spoken and written English.
Hope that helps.