I was with the same problem, as far as I know achartengine doesn't support annotations currently (I asked on forums).
Android Plot and other free chart solutions mentioned here doesn't support annotations, the only one is AFreeChart which is a port of JFreeChart for Android, I am currently using it and it is awesome and has much more features than achartengine.
EDIT:
See here, I have modified ace source code to have annotations capabilities.