Last time I went to Munich was 2005.Then you could just walk up and buy tickets on the day, - only the final was sold out well in advance as I remember, though obviously things might have changed over the years. It's a nice tournament - really nice gardens and grounds, very compact, and all the practice courts are easy to view - very friendly place. Hope you enjoy it.
thanks a lot for your answer! I hope it's still the same then!