I'm not sure where the second poster got this (but thanks very much!), and I've not had a chance to try this yet, but I believe I can explain it:
After upgrading to DNN5, be sure to include the ProviderName attribute into the dnn:ACTION control of your containers:
<dnn:ACTIONSrunat="server"ID="dnnACTIONS"ProviderName="DNNMenuNavigationProvider"/>
Go to your site (probably using FTP if it is not on a local computer) and find the *.ascx file that represents your skin in use. Likely places for your skins:
C:\inetpub\wwwroot\admin\Skins\skinname.ascx (for admin/host skins)
C:\inetpub\wwwroot\Portals\_default\Skins\_default\skinname\default.ascx (for normal users)
Back these files up!!! Then, inside these text files (i.e., it is OK to edit using notepad) ensure that wherever you see
<dnn:ACTIONS
there is also a
ProviderName="DNNMenuNavigationProvider"
entry, as displayed above.
However, for my site, there was no instance of "ACTIONS" that I could find. Hmmmm.