If I understand correctly, Rob, you're referring to spring-loaded folders, which is one of the four most annoying little things (*) in Mac OS X.  That's why BookMacster has a special feature to deal with this.  If you hold down the shift key as you enter the BookMacster Content Outline, folders will not spring open.

Actually, we designed this for drag-and-drop within BookMacster, but I just tested it and confirmed that it still works for dragging in URLs.

Although dragging from Safari's address bar as certainly supported, there are several other ways of adding bookmarks to BookMacster directly.  They're explained on this Help Book page.  My favorite is the Anywhere Menu.

* * *

(*)  Here are the other three most annoying little things about Mac OS X destroy my train of thought at least twice a day…

• The function key which, if accidentally clicked, activates - ick - Exposé.
• Accidentally double-clicking the title bar of a Finder window minimizes it to the Dock.  (This is on by default but can be turned off in Finder's Preferences.
• Swiping too hard to the right or left in Safari or Xcode will swipe the document you were looking at away and replace it with the previous or next document or tab.