I actually recommend not enabling those – the errors you’re interested in are in the Apache error_log. You can find those in $HOME/logs/error_log.
Enabling those becomes a security issue, as anyone visiting the site could end up getting a lot more info than they should be able to see should an error occur.
That said, if you really wish to enable them, log into Virtualmin, choose the domain in question, then click Services -> Configure PHP5 -> Error Logging.
In there, you can set "Display error messages" to "Yes".
-Eric
that is if you choose to run mod_fcgid
in VM Pro you can also choose to run under mod_php and then you’d still have to go to /etc/php.ini
I also strongly recommend to read the virtualmin documentation at least once to learn a bit more about webhosting in general and the software you are using specifically