The driver currently requires a configuration (XF86Config) file. A sample file is included in the x11/video directory. You'll need to make sure that it's readable as /etc/XF86Config. This can either be done through the BMOD (section 10.20) filesystem, or by using the FreeBSD or Linux filesystem components.
This file is a subset of a standard XF86Config file and can only contain the Monitor, Device and Screen sections. The easiest way to get things running is when you already have an XF86Config file for that computer/monitor configuration. In that case you can just strip out everything from it but the sections listed above. Otherwise you'll need to make one from scratch, or from a sample config file from an XFree86 distribution.
Currently the x11video driver only uses the default (first) entry in the Screen section. Unlike XFree86, there is no way to change or specify color depths or resolution at runtime.