You should have received an email this morning telling you that your preorder had been charged, and providing you with a serial number and a link to download Stardock Central.
If you purchased the game with the same email address as you use for your forum account, you should simply need to log into SDC with said account, and you'll find GC2 listed under 'purchased products' in the Games section. Simply select it and click install, and click the [+] next to it and do the same for the modules (including DA).
If you did not purchase with the same email address as you use for your forum account, you will need to register the serial you were provided to your forum account. To do this, run SDC, go to the Tools menu and choose Register Products. Choose Games on the left, then find GC2 in the drop-down list, enter your serial and email address and click Register (You may need to do the same for DA). You should then be able to download as above.
GalCiv2 Forums