Follow the messages ... and you're done.
If you like a more web-like approach, a good start point can be
-http://www14.software.ibm.com/webapp/download/downloadaz.jsp and click
+L<http://www14.software.ibm.com/webapp/download/downloadaz.jsp> and click
"C for AIX", and follow the instructions.
=head2 The usenm option