how to forward port 21 using an actiontec dsl modem?

Answer 1

http://portforward.com/english/routers/port_forwarding/Actiontec/GT704-WG/BitTorrent.htm
just ingnore all the stuff about bittorrent, and replace the numbers with 21 and names with FTP

you could also set it into modem mode and use it as a pppoe connection, the pppoe user/pass is the same as your email account.
you should then invest in a router, which are ment for that sort of thing.

Answer 2

The last Actiontec modem I used was a complete pile of garbage -- it didn't properly implement port forwarding or triggering -- I ended up putting my computer in the DMZ.

If you have a router, use that to do the forwarding (Actiontec doesn't work worth sh*(T)