Hi,
On ACS you need to go to SYSTEM CONFIGURATION > LOGGING and make sure CSV RADIUS Accounting is ticked.
On your switch, if for example you wanted to monitor system events you would have similar:
aaa accounting system default start-stop group radius
radius-server host [host_IP_address] auth-port 1645 acct-port 1646 key [shared_secret]
You can then view the accounting logs in ACS under REPORTS AND ACTIVITY > RADIUS ACCOUNTING
Depending on what you are trying to do, if its just device level stuff you want to log such as commands entereed on switch etc you may be better off using TACACS+.
HTH
Paddy
