I suppose you have a config like that:
Terminal Server is inside and I suppose you have a static public outside IP address.
Example:
static (inside,outside) TS-Public-IP TS-Inside-IP netmask 255.255.255.255
access-list internet permit tcp any host TS-Public-IP eq 3389
access-group internet in interface outside
Just check if there is allready an access-list on the outside interface.
=> show access-group
If there is allready an outside access-list just change the access-list name "internet" to the name of your existing ACL.
Cisco PIX Configuration examples:
http://www.cisco.com/pcgi-bin/Support/browse/psp_view.pl?p=Hardware:PIX&s=Software_Configuration
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a0080094466.shtml
sincerly
Patrick