Just curious, did you also open the respective ports on the Macs?

If so, then skip this part. In general (since I don't know which version(s) of OSX that you're using), you'll need to open System Preferences, select Sharing (in the Internet & Network section), and click on the Firewall tab. Here, you can add an entry for the port(s) that you'll need to open.

Second, did you check your firewall rules to ensure that the respective ports are also open on your Linux boxes?

If so, then skip this part. Assuming that you use iptables for your firewall rules, then you can list the rules in effect with the following: iptables --line-numbers -nvx. Depending upon how your firewall is set up (ie does it have default deny-all or allow-all rule?), you might have to explicitly add a rule to open the respective port(s). You'll definitely have to add a rule if your firewall has a default deny-all rule.

Third, assuming you configured your game server to work with either inetd or xinetd, did you check to ensure that inetd or xinetd is running? And did you check your /etc/hosts.allow to ensure that the respective ports are also open on your Linux boxes?

If so, then skip this part. Open /etc/services in your editor of choice. Check to ensure that an entry exists for the port that your game server uses. Add an entry if it doesn't already exist. Open /etc/inetd.conf or /etc/xinetd.conf in your editor of choice. Check to ensure that you have an entry for the game server, and check to ensure that you enabled it. Open /etc/hosts.allow in your editor of choice. You should also check here to see if you have a default deny-all rule. If so, then you'll need to also add an entry for the respective port(s).

Fourth, although you didn't mention whether you were using a switch, route, or some other network appliance, you'll need to check to ensure that the device isn't blocking any ports.

Fifth, you should use a sniffer like ethereal or tcpdump--both on the Linux boxes and the Macs. This way you'll be able to verify whether or not the network traffic is at least reaching the destinations.