AI used the most common recipe for modeling chocolate I could find (7 oz of chocolate melted in a double boiler, mixed with two tbs corn syrup, refrigerated for two hours) and it crumbled right in my hands. How can I prevent this and what did I do wrong???

Let it sit until morning and then knead it a bit if you're going to use it today. It looks funny but the heat from your hands will make it usable. HTH

You maybe did nothing wrong at all. Different kinds of chocolate take different  amounts of corn syrup.

Go to www.wickedgoodies.net for LOTS of help and advice.  Get her  free book preview because it explains exactly what happened for you.

What chocolate did you use?

AI used chocolate chips. Thank you for the help! Round two here we go!

Seriously, don't refrigerate it.  The condensation is probably what made it crumble.  I use modeling chocolate exclusively to cover cakes since it tastes so much better than fondant.  I personally use about 1/2 the corn syrup called for in most recipes.  Once it's made and cool enough to handle, knead as much of the excess oil out as you can. Then just let it sit on the counter overnight, or at least for a couple of hours.

Once mc is made, you handle it just like fondant and I never put fondant in the fridge either.