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out of curiosity, when you did the drag and drop did you sent your TopSites to edit mode; that is is the page set to allow you to move pages around or delete them? I ask because I initially was not able to add pages if it was not set to edit. Also, try and open TopSites in a different WINDOW, not a different tab and try and drag and drop while in edit mode. At least for me, that has worked fine so far
 
I guess it works with the TopSites page in a new window. I previously tried having the page I want to add in a new window and the TopSites page as a tab in my main window, but that didn't work.

Hopefully Apple will make it work better in the final release.
 
Go to the page you want to add and drag the address down to the Top Site icon in the bookmarks bar.
 
Open edit mode in Top Sites and then open the site you want to add in a new window and drag the little url icon into the Top Sites page. Voila.
 
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