FWIW, my Help/Updates does not work. My ACR is 8.0 and each time that ACR has been updated during the last year I have tried to update it through Help/Updates and each time I have received in the Application Manager window that opens the text, "Your applications are all up to date." I suppose it is possible that the fact that I have an updated copy of LIghtroom (5.6) is confusing the Application Manager.
Because I do all my Raw processing in LR it does not really bother me that the PSE 12 ACR plugin does not update.