Membership "add to member level" not working

I updated the Membership Premium plugin to Version 3.5.0.6 and now when I go to "All Members" it does not allow me to add new subscribers to a member level. There used to appear an "Add" option when I scrolled over the member level column, and now nothing appears. Help!!! This is urgent, as we run a membership based association and this issue affects us greatly.

Thank you!
Scott