if you use windows, hit start then run. type "winipcfg" and hit ok. Then use the pull down menu to select your adapter and the "adapter address" below the menu is the mac address. It "release" then "renew" to get on the spot information.
The advice above is good for internal modems. But since they are usually of external type, just look on the back or buttom of that plastic box with lights the provider gave you. It should be somewhere near a serial number. It is a 12 digit number.
winipcfg only gets u the mac address of adapters that are installed in your systm...ppp...NIC's...that sort of thing...either the mac will be on a sticker on the modem listed as a mac add. or a mac id or possibly an ea ...if u don't have any of these listed and if your really lucky your modem may have an internal dhcp in it, then u just got to figure out the internal ip and then get into the modem via a web browser and the mac will be listed in there