You need to be able to trigger an Advanced Action on mouseover of an object. This Advanced Action can be set to SHOW any number of slide objects that start out as hidden.
There are two ways to trigger an Advanced Action with a mouseover event this in Captivate.
One way is by using a rollover slidelet and using the Roll Over event of the slidelet hit area. You can move the part of the slidelet that normally appears on rollover somewhere off stage where it doesn't get seen. All you really need the slidelet for is to trigger the Advanced Action.
The other method is to use the Infosemantics Event Handler widget. It allows you to trigger Advanced Actions on rollover or rollout of any object on the slide. (It also allows you to use many other mouse events to trigger actions.)
You can learn more about it here:
It's the third widget from the top.
Rod's explanation is perfect, and I hope you are aware that rollovers are not supported for HTML5 output.
http://blog.lilybiri.com/events-and-advanced-actions allows you to download a list with all available events in CP that allow triggering actions.
The dots that appear when rolling over, should be shape buttons that can trigger any action, like going to a slide where a weboject is inserted or opening an URL directly.
Often wonder why people prefer spending hours and hours instead of asking advice?