[PhilJ]

The popupanywhere module has been tested and it does work, but I generally don't recommend that module, as promotional popups are evidently annoying.

The necessary code is already added to...

/skin/reboot/customer/content.tpl
/skin/reboot/customer/service_js.tpl

Maybe clear your cache, or check that you've correctly configured your popups.

There's a timed event modal popup in the template settings > misc that you can use for similar purposes.