You don't need to use multi/sub materials... use a single "Forest Material" for each item on the "Geometry List".
The "Material" slot, and all parameters included in the "Properties" box: type of geometry (Disabled/Template/Custom), width, height, z offset, center disp., probability and material are defined locally for each item selected from the list.
The randomness is done not only changing the material ID as Forest 2, but taking a random element from the Geometry List, and creating a tree with all these properties. So, you would create a Forest with these types of trees:
Sample Tree 1: width: 2, height: 3, geometry: one plane, prob: 30%, material: 'tree1'
Sample Tree 2: width: 4, height: 8, geometry: two planes, prob: 70%, material: 'tree2'
Where 'tree1' and 'tree2' are "Forest Material 3", not multi/sub.
And don't drag and drop any material to the object, because you overwrite the multi/sub material that Forest creates for itself. Use the "Material" slot, picking them from the material editor.
We would create a Videotutorial to show how to use materials, but we will need time to complete it...