Sorry I didn't mention that I am using crystal reports in .NET framework. I haven't found what you mentioned Top N / Sort Group Expert.
When I open a crystal reports file, there is a Crystal Reports menu on the top. Click Crystal Reports, there is a Report option. There are lots of option under Report. The menu structure is:
Crystal Reports
Report
Select Expert...
Selection Formula
Record...
Group...
Formula Workshop...
-----------------------
Section Expert...
Group Expert...
Group Sort Expert...(Grey)
Hierarchical Grouping Options...
Record Sort Expert...
-------------------------
Report Options...
Set Print Data and Time...
Summary Info...

Any idea will be appreciated.

Thanks,

Bruce