Who is Online was a hack added to the previous version of the software. I upgraded to a newre version yesterday and because it uses some php routines the hack will not work with the new version.
Until a new hack is available then you will have to do without. The next version of the software promises to have a Who is Online function built in so I may just wait for that. Probably available in a few months.
In the meantime enjoy the faster response of the new version.