If you are using Office2007!
1. Select the Fly-in object.
2. Select Animation from the toolbar and select Fly In from the drop-down list next to the Animation option
3. Select Custom Animation.
4. Select Start, Direction and Speed from the displayed options at the right side.
5. Click on Fade Text Box.
6. Select Fade from Animation drop-down list.
7. Select Transition Speed Slow
8. Put check mark in the Automatically After 00:00 tool bar at the right end.
9. You may change the time to 00:04 to further slow down the Fade action.