I'm using XP, but it should be similar:
Fire up explorer. Tools->Folder Options->File Types, then scroll down the listbox and look for DBA. Select DBA, then below the listbox there's a button labeled "Change..." so click it. In the new dialog box that pops up, click browse and look for your darkedit executable. That's it.
If that doesn't work for some reason, just right click any DBA file, Open With->Choose Program. Click Browse and look for the darkedit exe. Don't forget to tick the "Always use the selected program to open this kind of file" check box.
If that still doesn't work, right click a DBA file, Properties, then click the "Change" button, Click Browse, look for the darkedit exe.
Follow my vague instructions and you should be all set.