I have an ADSL access point modem(D-link DSL 2640T) at my home that Valid IP address is assigned on it.

Also two PCs connect through the ADSL Modem access point to Internet.

I want make remote desktop connection to one of the PCs from Intenet outside my home.

Meanwhile the two PCs get private IP from ADSL access point Modem DHCP.

Could you please help me how can I do it?

You could use a program like this:
https://secure.logmein.com/home.asp

I have tried it and it works well to connect remotely.

If you really want to use Remote Desktop specifically I recommend going to dyndns.org and getting a dynamic DNS set up so that you can connect via a hostname that doesn't change. Next you will need to go in to the setup page for your DSL modem and set up IP Passthrough to your router. This will dump all traffic destined for the DSL modem directly to the router. Now on the router configure port forwarding for port 3389 to go to the internal IP of the machine you want to use Remote Desktop to connect to.
Now you can RDP from outside your network to your dynamic host name and connect to the PC on your internal LAN.

Getting the personal version of LogMeIn is much easier and probably less risky. So I recommend following Scooby's advice. But if you MUST use RDP, then that's how you do it.