You can't actually telnet to the outside interface of a PIX unless you come in over a VPN tunnel, even with the command you entered. This is part of the security of the PIX, telnet is NOT a secure protocol (passwords sent in the clear), so you don't want to be telnet'ing to it over a public network.
Your best bet is to use SSH instead of Telnet, then you can SSH directly to the outside interface.
Here's how to set it up:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_60/config/commands.htm#xtocid75
and here's a link to a free SSH client called Putty:
http://www.chiark.greenend.org.uk/~sgtatham/putty/