Make sure the nvidia driver is loaded, as with the linux module you can load it with
kldload(8):
# kldload nvidia
To load the module at boot, add
nvidia_load="YES" to /boot/loader.conf (Create file if it doesn't exist).
Then make sure you're using the nvidia driver and not the nv driver.
You can change this in your
xorg.conf(5)
If that doesn't work, post your xorg log file. It's located at
/var/log/Xorg.0.log
Please post this between [code] [/code] tags.