There are 2 numbers you need. The first is the steam activation, which gets it installed and on your computer. The 2nd is the product key. To get that right click on Fallen Enchantress in your steam library and select the "View CD Key" option. That's the key you need to enter in the Stardock activation dialog.
This is kinda confusing?
True, with Legendary Heroes the activation key is being removed from the Stardock registration dialog. Because we have multiple ways of getting FE, and people have gotten it through multiple channels we need it to tie people to their products. That's what the stardock activation key is for. But one fo the big advantages of having LH be always redeemed with Steam is that we can cut that hassle out.
Why do we need a Stardock registration at all?
Valve is a good company that protects their customers privacy, as they should. Because of that they don't tell us who you are, even if you have purchased a game from us. We have no way of validating who you are if you want to use support, if we want to email you about a patch, or if (god forbid) something happens and Steam goes away. In that case we would want to be able to give our games to customers who have purchased them, but without a registration of who those people are we are lost. The Stardock registration isn't DRM of any sort.