Hmm... perhaps try doing some radiant quests, nothing major, then check back after you've done a few. If she's not there by then, try reloading an older save before you told her to leave. Also, is serana still your follower?
Hey guys I can't find the castle or place I first met Valerica I have ran around the Soul Carin a couple times. Is there a quest or something I need to obtain before I can tell Valerica that I killed her husband
All you need to do is walk up to the entrance of the castle and she'll be outside using the Alchemy lab, talk to her after killing Harkon and the last chat option will be to tell her he died. She than will gather her things and appear at the castle.