Select how many columns and rows you want in your poster. You don't have to use all of them and make sure that you have enough so that if you want to expand cells that you have enough for all your photos.
Setting either value to zero will create a free form collage. When you close this dialog you will be able to add all the cells you want using the Insert New Cell dialog from the menu.
This is the margin that you want to have around each photo. Set it to 0 if you want the pictures to sit right next to each other. When you add enhancements such shape and border properties there will be a slight margin between these photos automatically.