What does "activated" DOI number mean?
Because DOIs are often fairly predictable, and it's useful to have a single ID, many publishers assign them at an early stage of publication, well before the article is online. At this point, though, they're just a string of digits and don't really "become a DOI" until they're entered into the CrossRef system. I'm guessing this is what your publisher means by "activated".

When the publisher has submitted the metadata to CrossRef, the central system at dx.doi.org now knows that doi:10.1234/5678.910 should resolve to www.example.com/journal/article, and the link can start working. Once the metadata is submitted, the DOI should be active fairly quickly - within a day, if not sooner, is my experience. However, some publishers delay submitting this metadata for a while even after the article is first published online in an 'early view' status or similar - I have no idea why, but I've seen it done. 

Right now, there is nothing technically stopping publishers submitting the metadata before making the article live, but it's recommended to do so at or shortly after publication - so your DOI probably won't work until shortly after the article is posted online.

It would be nice for DOIs (especially for high-profile material) to be live at the moment of publication... and it may happen fairly soon. CrossRef recently held a consultation on how best to do 'advance registration' of DOIs for unpublished material, and from comments I saw at the time I suspect the responses were fairly positive. This suggests we'll see a more robust approach to this in the near future, with DOIs that are live and resolve to something in advance of publication.