You're going to have a hard time cutting foam with that laser, you can probably cut the black foamboard if you can find it, but not the white. Take a look at the mpcnc thread in the 3d printing forum here for a lot of discussions about the alternatives. (Needle cutter, Spindles)
I built the mpcnc and initially started with a needle cutter, It works well. I've recently replaced it with a tiny bit for a spindle and am having even better success. Ive also got a 2.5w laser that I can swap on to it in a few seconds, which is super useful for things like poster board and thin plywood.