This must be national tag day 
– in the other thread, the reader cannot get rid of tags, and in your thread you cannot add them.
Seriously – sometimes adding tags in Mavericks doesn’t take immediately. There is a delay. But, generally if you add tags to a document in Finder > Get Info or by using Tags in the Finder sidebar, and then drag that document into DEVONthink you should be able to see the tags. You are saying that doesn’t work for you – ?