Generally, LMCE should see it on its own and present it to you as an option... eventually, it may take some time before it sees it. But if after several hours it still hasn't done this, then there are a few things that may be preventing it or that you can use to stimulate it:
1. If you have XP's firewall on this will cause any number of problems for connectivity... turn it off, it is on your Internal LMCE network so it is already protected by LMCE's firewall.
2. Try disconnecting the PC's network cable for a few seconds and reconnecting it... this will retrigger DHCP, and that may cause LMCE to redetect it.
3. When you first booted this PC on your LMCE internal network (assuming no firewall), LMCE should have prompted you to add it as a "Fileserver". If you said No, and don't ask me again, then it will be in your ignored list. In the web admin, Advanced->Configuration->Unknown devices, find the PC, and use the Unlock button.
4. Reload the router
5. There is a Samba scanner script that possibly could retrigger the process....
6. Make sure that your XP machine is in the LINUXMCE workgroup
7. Worst case, we can add the share manually, but it would be better that you troubleshoot why it isn't seeing the server/shares automatically...