Go into your arrangements properties (e.g. by pressing the "E" key). Among other things, you will find a little drop-down list with different options for the time signature ("beat type") here. Naturally, this merely adjusts the mode and layout of the program. If you're basing your work on ready-made sampling content, you will need the suitable 3/4 material. I hope the future will bring us loads of Magix waltz soundpools. ;-)