Version:            (using Devel)
OS:                Linux
Installed from:    Compiled sources

It isn't possible to drop plamoids on a panel, that has the auto hiding option enabled, because the panel doesn't pop up when you move the plasmoid in the area where the panel is. 
To add a plasmoid, you have to disable the auto hiding first. Afterwards you can add your plasmoids and then enable the auto hide feature again.

This sucks pretty much though^^

Thank you

pano