The BMW NAV III comes with a power supply that is required for you to operate the device attached to your PC. It also comes with what is not a fairly common cable, as most mobile phones use the same cable for charging and data transfer as do many digital cameras.
I think the cable is USB to mini-USB, but i do not have the Nav III with me at present.
If you do not have the power supply, you may have to get that from Garmin, as the device end is specific to the NAV III.
Gamin sells Map Updates on their web site, http://www.garmin.com
The also have a number programs which are free to registered device owners. They have versions of Windows and Mac, and as I look on my MacBook Pro which runs OS/X 10.6.7, I have the following:
Garmin WebUpdater (as I signed up (paid for) quarterly web updates for the maps.
The power supply is Garmin # 320-000171-12. You can probably take the NAV III to Radio Shack, Best Buy and get the proper data cable, but you will most likely find the power supply only from Garmin or E-Bay.
Garmin gives out most programs free, although they charge for maps. They do require your NAV III to be turned on and plugged into the USB port on your computer before they hand out anything. They do take care of giving you compatible software that way. Their web site has plug-ins for IE, Firefox and Safari so that they can control the NAV III.
The NAV III also came with a weighted pad and a automobile charger/speaker, and a remote control so you could use it with your car.