Angela - you don't say which make and model - I have a Miele W/Machine - it too has three compartments - a tiny one with a lid where we pour the Fabric Conditioner. The other two are marked (I) and (II) and I guess one is for pre-wash stuff and the other for main wash powder - Powder for the main wash will go into the biggest chamber which on my machine is number (II) in the middle.
You can always do a test - put the powder in the one you think it should go in and run a wash without any pre-wash - if the powder's gone at the end and the washing is clean you've solved it.
