Although Graham is correct here's a hack of a workaround that I truly DO NOT
recommend be used extensively but it may satisfy a limited need...
Select the word/phrase you want to serve as the rollover & create a
Bookmark,
Keep the text selected & create a Hyperlink to its own Bookmark [because you
have to hyperlink to *something* & that keeps the user from jumping
somewhere else if they do click it] & use the Screen Tip button in the
Hyperlink dialog to create your message,
Then reformat the hyperlink if you wish so it doesn't "look" like a
hyperlink.
One minor down side is that the rollover Screen Tip will include the
"Control+Click to follow link" message which I don't know how to eliminate.