How can I change the default message for protected content?

HI,

I can't see anywhere I can change the default message that is displayed for protected content, i.e.: The content you are trying to access is only available to members. Sorry.

I have tried wrapping messages in the shortcodes for guest but this doesn't seem to work, unless I need to say any visitor not logged in is a guest but unsure where to do that..

thanks!
Chelsey