Click on the slide to deactivate the chart.
Click once on the chart to select it.
Slide Show|Custom Animation
In the animation task pane, click Add Effect. Choose Entrance and Appear.
Right-click the animation in the task pane. Choose Effect Options.
Choose the Chart Animation tab.
Select how to animate the bars there -- by series or category or element.
The grouping options available depend on a combination of which chart and
which animation you've chosen. (That's why I had you use Appear above --
just about everything is available with it.)
If you choose series/category or element, a downward-pointing chevron
appears. Click it to expand the animated elements -- this allows you to
specify different starts and durations for the individual elements. See
http://www.echosvoice.com/individualbullets.htm#IndividualBulletedTextAnimation
for screenshots of expanding the animations on the task pane.