Go to NAT, right click and add interface, change the public-external to
private-internal (radio buttons) or vice versa.
NAT is smart enough to 'guess' that a NIC is likely private-internal and
a modem is likely (almost always) public-external but if it guesses wrong
you can always choose the other radio button.
A further note on this. It depends on what OS you are using. W2k RRAS
console does not "see" the internal interface as an option, so you need to
use the netsh command. This has been fixed in W2k3 and you can now add it
from the MMC.