How do I successfully set up a Microsoft mn-100 router so that I can
access my FTP server from the internet?

It has the latest firmware upgrades, and we are using Time-Warner
Cable internet access in Minnesota.

Open port 21 (or whatever port you set your FTP server at) and allow
traffic in and out

Yeah, I'm familiar with how to do it on any other router, but I've
spent a while trying to get the same thing to work on the mn-100. 
Needless to say, I haven't yet succeded.

From the manual: 
http://download.microsoft.com/download/B/6/9/B69C956C-85D9-4641-AA6F-1548391E0967/users_guide_100.pdf
To configure persistent port forwarding
1. Open the Base Station Management Tool, and then click
Security.
2. On the Security menu, click Port Forwarding, and then click
Set up persistent port forwarding.
3. In the Description box, type a description of the server field.
(This step is optional.)
4. In the Inbound port box, type the inbound port to which data
packets sent from the Internet to the server will be passed. The
inbound port can be a single port or a comma-separated list of
ports or port ranges. For example, you could type 4-25, or 243,
or 10, 24-50, 74. You are limited to 256 characters.
5. In the Type box, select the protocol (UDP or TCP) for the port.
6. In the Private IP address box, type the private IP address of the
client computer that is hosting the server.
7. In the Private port boxes, type the private port on the server
that the data will be sent to. To identify the private port number,
consult the documentation for your server software.
8. Click Apply to save the changes you have made.

Be aware that you may only be able to connect FTP using PASV/Passive
mode through the firewall.

Yeah, that's what I've done already.  I "forwarded" port 21 on the
router to my server's internal IP address at port 21.

However, I never tried the "passive mode" thing.  I'll see if htat
fixes the problem. (may take a couple days because the problem is
actually at a friend's house.)

By the way, thanks for your help so far! :)