That link is a "how to" for
FreeBSD.
It also assumes you have a working Apache/PHP environment. Do you? e.g.: if you are using OpenBSD's built-in Apache, have you read and followed the instructions in FAQ 10.16?
While I don't use symon, I'm sure that even a symon user will not be able to help you without the specific configuration information from
your system. The "how to" you referenced descirbes directories that do not exist with OpenBSD, you know.
Try posting your configuration information. If you copy/paste configuration files, please tag the content with [ code ] [ / code ] tags, so that it remains readable.
Lastly, have you seen the package installation message? If you missed it, try:
$ pkg_info symon
and then:
$ pkg_info -L symon