Can't host games on the LAN with my Mac?

See if this helps...

The problem seemed to be that the games don't properly handle my PC having multiple network adapters. They then connect using the wrong network adapter and only see that adapter's own network/subnet.

In my case this was caused by the Virtualbox network adapter, although it also seems to be common with other software adapters such as Hamachi. The simplest solution I could find was to disable the problematic adapter before playing any game. Other possible options are to disable all other network adapters except the one for your current network or to correctly setup the priority of your network adapters.

The priority of the network adapters can be set by following this procedure in Windows Vista/Seven (source):

• Open 'Network and Sharing Center'
• Choose 'Change adapter' settings
• Hit ALT on your keyboard to show the menus
• From the 'Advanced' menu choose 'Advanced Settings'
• Under the 'Adapters and Bindings' tab select your proper network adapter from the 'Connections list'
• Use the green arrow on the right to move it to the top of the Windows Network Adapter Priority List
• Click 'OK' to apply the new settings and you are done!

https://superuser.com/questions/610733/networking-games-cant-see-join-anyone-elses-lan-servers-unless-i-host

Are the games on both computers running the same version of the game?

You may need to check your router's configuration settings.

If this and @BDAqua's suggestions do not help, then contact the developer of the game for assistance. Cross platform game play can be complicated and may not always work reliably due to the differences in operating systems. How well cross platform play works depends on the quality of the networking code used by the game.