If you're downloading legal non copyrighted material, it sounds like the problem has to do with port forwarding. It's really easy to setup, you need to know:
The port your client is attempting to access
the computer's ip addess
the router's ip address for the menu page (usually 192.168.2.1 or something similar)
The Mac's ip addy can be found in the network window under the computer preferences and the port the client is using can be found under the client's options.
You should also setup a static IP if this is a stationary computer, or else you may have to change the port's IP in the router when you restart.
Log into the router, there should be a section for port forwarding. In this section you'll want to create a new service for the computer's IP and the client's ports. Test the port in the client and it should be good to go.
If you're downloading copyrighted material illegally, you probably shouldn't and this post will be deleted.