By default, "clamav.conf" or "freshclam.conf" are saved to /etc directory. If you can't find them there, run the command "locate clamav.conf" and note the location if the command locate outputs it.
I have never used clamsmtp, so I won't be of much help here. The documentation that comes in clamav-0.88.1-1.i386.rpm and/or clamav-0.88.1-1.tar.gz file contains good amount of documentation on how to setup clamav and postfix.It is exhaustive enough, if I rememeber it right. I remember, it even comes with a test EIKAR virus, which you can experiment with. You can setup clamav.conf and postfix configs and then run a telnet session to hook onto clamav and postfix as, clamav listens on port 10025 (If I remember it right). You can send a test mail containing the test virus (EIKAR) from a console and see the log output from clamav disaplyed on another console screen. This is the best way to see clamav in action and to know that it is indeed working. Oh ya ! All this is explained in the documentation !!
You might wanna do the same with clamsmtp. I mean, use telnet to send an email containing virus and watch the output logged by clamav on a console screen !
If you are not able to send and receive mail, chances are that you messed up your postfix/sendmail config files. Oh, BTW, the documenation that comes with clamav also explains how to setup postfix/sendmail. May be, you should do a bit of reading ! Goodluck !!