rejieah,
Go to:
http://www.fida.com/manuals/h9kv1.pdf
This is the manual for your ADSL modem/router. You'll need to access via a web browser on a pc attached to this router (see page 12). Look at page 18, which describes NAT settings. You want to set up NAT to forward TCP Port #21 (FTP) to the IP address of the PC or server on your local network running the FTP server.
You shouldn't need to mess with your DNS records to do this. The only reason you'd need to change your DNS records is if you wanted to redirect FTP traffic to a different physical internet address. If all of your PCs are using this router and the same external IP address, you should be able to setup a NAT port forward on the router.
- John