Thanks Oli, I hope you're able to complete the tutorial.

As for not being able to make the bottom area Planar, please look over the previous post: http://moi3d.com/forum/index.php?webtag=MOI&msg=5530.29

If the bottom profile curves are not flat, not touching, or not tangent, then Network may not make a surface with a planar edge.
If the curves are both tangent and touching, then you can make them flat in a side view by moving the handle bar until a tab says "Flat".

If nothing works, you can also Boolean Difference, or use Trim to cut a plane away from the bad edge.

Good luck!