I solved the installation problem by following instructions i found in the pre-order thread. here is how I did it:
downloaded and installed the DLC (in my case from Gamersgate) to the download folder (instructions were to use desktop). you will find a folder there called "HOI3". inside are two files (gfx and mod) plus uninstall stuff. I just copied the two above files (gfx and mod) to the main game folder (which should be a copy of unmodded HOI3/SF).
use the laucher, now you can choose between default (vanilla game) and Dies Irae. runs just fine now.
DD, Danevang, you guys did a splendid job. A little disappointed you get standard names (interceptor I instead of BF/ME-109, for instance), but I take it this will be taken care of eventually.
hope this post helps for anyone having trouble installing the DLC.