On Feb 20, 2011, at 9:25, Paul Cartwright <paul@...> wrote:

> in a super-user terminal I get this:
> # synaptic
> Invalid MIT-MAGIC-COOKIE-1 key
> (synaptic:19493): Gtk-WARNING **: cannot open display: :1.0

Normal.

>
> in a user terminal I get this:
> $ synaptic
> bash: synaptic: command not found

Also normal.

>
> from the debian menu, clicking synaptic does nothing.
>

Can you check if you have kdesu or kdesudo from trinity installed?
Also, if you have gksu installed?