To format the values in your Reporting Services pie chart on the outside of the chart, first right click on your pie chart and select “Show Data Labels”. If you have not configured Values and a Category yet you will need to do this.
Now select the pie chart to reveal the Properties for the Category. Scroll down to the Custom Attributes property group and located the PieLabelStyle property. Set the value of this property to “Outside”. Thats it.
Read more about it here http://technet.microsoft.com/en-us/library/dd220583.aspx.