To set a module as the top-level module, right-click on the module and select "Set As Top Module".
When attempting to set a System Generator module as the top module, a pop-up appears with the following message:
"Changing the top module setting....out-of-date. Do you want to continue?"
When I click "Yes", nothing happens; top module has not changed.
This problem has been fixed in the latest 10.1 Service Pack available at:
The first service pack containing the fix is 10.1 Service Pack 1.