Be sure to grab the latest patch. This will update your game version to 1.11 allowing you to play online and also fix some bugs.

Patches & Updates 

Medal of Honor Allied Assault™ UK & US 1.11 Patch

Highlights:
1. Stand-alone game server that runs without CD
2. Fix for the dial-up networking problem when playing single-player
3. Fix for in-game server browser always auto-refreshing
4. Fixed lag when a bomb was planted and the entire planting team was killed
5. The multiplayer dial-up connection fix