Update notifications not appearing in plugins or menu bar.

Have updated to 3.1 and am having difficulty with the update notifications plugin. When I install it nothing happens. I remove the previous file from muplugins and put the latest folder in to plugins but there is no sign of it in the dashboard area and the nag at the top remains asking me to install the latest version . It's not in plugins either.

I am using Nelo 2.1.2 theme. Any ideas what I might try

Thanks