Accessing an external hard drive plugged in an Airport extreme ... from the web ?

Hi.

The question is included in the title.

I'm looking for a tutorial (... for dummies), to get an access to my hard drive, when I'm out of home (so I guess it may be possible from the web).

The web access is OK and I can access the hard drive at home by WIFI.

I tried to find out with Airport Utility (installed from my Macbook) but I didn't succeed.

Thanks for your help and assistance (and sorry for my english too).