first thing to do is make sure you're running v1.05:
http://www.lspn.net/downloads/linkspc/Links2k3_105_Final.exe
then point your registry to the correct server:
2k1:
http://test.lspn.net/test/LSPN_Main_Site_2k1.exe
2k3:
http://test.lspn.net/test/LSPN_Main_Site.exe
run internet explorer, internet options|security, and add
http://www.lspn.net as a trusted site.
within links, connection settings, uncheck 'use system internet settings', check 'connect through lan or connect manually', and make sure use proxy is unchecked.
if it were easy, anyone could do it. (8
edit: oops, i just noticed your post is in the lobby games forum. if you're asking about match maker, here's everything you need to know:
http://www.portforward.com/english/applications/port_forwarding/Links/Linksindex.htm
edit #2: please excuse all the typos. i just got back from a college baseball game, and i'm frozen to the bone. the fingers aren't working correctly.