While most downloadable drivers will come with their own installation routines or scripts, this general rule should always apply. In case you ever need to install a driver manually, or if an installation routine fails, it can be handy to know.
User-installed drivers (including those for audio, video, scsi, gameport, whatever) should be placed in:

/boot/home/config/add-ons/kernel/drivers/bin

Be-provided drivers are located in the corresponding system-level directory:

/boot/beos/system/add-ons/kernel/drivers/bin

Once the driver has been placed there, create a link to that driver in an appropriate subdirectory of:

/boot/home/config/add-ons/kernel/drivers/dev

(similarly, links to Be-provided drivers will appear in the corresponding system-level directory).
For example, to install a video driver, you would place place the driver in bin and a link to that driver in:

/boot/home/config/add-ons/kernel/drivers/dev/graphics

Never install drivers in the system-level parallels of these directories (under the /boot/beos hierarchy).