Enable it for what? Oracle maybe? You probably need to install libaio and libaio-devel and relink if for Oracle.
On RHEL4 for a recent project Oracle specified:
libaio-0.3.105-2
libaio-devel-0.3.105-2
I just found this link that talks a little about Async I/O for Oracle:
http://nextre.it/oracledocs/asincio.html