Pie Charts

For a class Im tkaing we have to do pie charts. One of our tasks is to change
the shape style of the chart. Everytime I try to change it, it doesn't work.
I click on the shape effects in the shape styles group. When I click on it I
go to bevel and click 3d options. There I am supposed to change the top and
bottom to circle with a witdh and height of 512pt, but it won't let me do
this. After I click on circle I put my mouse over the top and bottom button
to make sure it changed and it still says none. Am I doing somethign wrong or
is there a problem with excel. Thanks for the help.