After you dismiss Serana, she tells you she'll be waiting for you at the Glade, I went to the Ancestor Glade, but I can't find her [maybe I missed her]. :S Is this not the glade she was referring to?
After trying out one of the answers, I still couldn't find her. DO NOTE that the dawnguard quest has been completed.
Best Answer
Were you doing the mission where you had to go there? Have you tried casting Detect Dead to see if she's hiding somewhere? If you aren't doing that mission, she will either be at the vampire castle or at Fort Dawnguard, depending on which faction you chose to help; sometimes she will mistakenly say the glade line after you complete that mission, but she will still return to the base.