To do this in Visio 2000 do the following:
Select the visio object that you want to add the pop-up comment to with the
arrow tool.
Then go to the Insert Menu - and choose - Comment. Type in the words to
you want to pop up and close the dialog.
Now when you move the arrow cursor over the selected shape - you'll get a
pop-up comment.
Hope that helps,
darrin (msft)