If you make a V cut at several points along the curve, you'll end up with a pretty good curve. You'll have to be careful not to cut through the foamcore completely, or you'll just end up with several different pieces on your hands. I'd reccomend, if you take this approach, to get a tool that specialized in this (there are a few that have been designed to do this).
[edit]Looking over the instructions again, it doesn't actually look like there are any foamcore-mounted curves in the set. Probably not a bad idea to add this in to add a bit more structural support to the model, but nowhere do you actually need to use it.[/edit]
Magius out.

The best advice I can give when it comes to cutting curves in foamcore is to slow waaaaaaaaaaaaaay down. If you're patient it's actually pretty easy. The real trick is to cut it in multiple passes. Instead of the usual "3 cut rule" double that to 6 cuts until you break all the way through. You'll find that with each successive pass, the curve will get easier to cut.

Break through the paper layer as your first priority and take your time with it. Making those first 2 or 3 cuts in the shallow part of the foam demands a slow handed approach. After that its pretty easy.

Thanks guys!

Magius - I have some circle cutters but they didn't really work for partial curves. Do you of something else that would work better?

Here's to hoping my freehand cutting skills can handle a curve. *crosses fingers* I've got all weekend so slow shouldn't be a problem.

I use my normal utility knife to break the paper on both sides. Then switch to an exacto knife. Which rather then pull cutting like normal, I mostly just push down, move over, push down... repeat infinitum.

But the thing to remember is even if it gets messed up and you have some foamcore middle pull out its really all hidden by the edging. So take care but don't worry if its not perfect.

Hmmm, X-Acto used to have a cutting compass - that held a #5 blade for cutting arcs/circles. *GOOGLE*

Mostly though, I would just recommend patience and practice. You may find it easier to cut in short lengths - making perpendicular cuts that terminate in the arc, then cutting between those cuts, so you are only cutting an inch or so at a time. Cutting from the inside of the arc also helps - any slips then go into waste space.

The Auld Grump

Something that might help in guiding the blade to make a curved cut that has varying radii is a metal french curve. Do a search for a 'metal french curve' and see if one or more will fit your need and budget.

Couldn't you just use 3/16" stock balsa wood and then cover with the cardstock? It would be so much simpler.