If you’re just looking for the solution:
1) The Mac player searches for and runs the 1.11 to 1.12 (1295) patch. Google’s the fastest way to find it– it’s hidden on the nwn site.
2) The PC player goes to nwvault.ign.com and downloads the 1.11 to 1.12 (1295) patch [even though they’re patched up to 1.13 (1409)], and the patch running program.
3) PC player unzips the patch running program and launches it. Select the patch and check the “ignore missing files” box. Ignore the scary warning. After a few minutes your version is set back to 1.12 (1295).
4) Launch multiplayer and find each other’s games.
The story follows.