Humm... maybe it is working. the ping response does not seem right. I'll try to see what it should look like on my 1200..
Humm, it should definately say "time=xxxms" if its working right. But it is "MiamiPing" you are using, right? I totally forgot its not simply ping.
Also you can attempt to ping the amiga from the mac or ping the mac vs the router, or ping the router from the mac... all useful info.
Under databases/DNS maske sure to put in 192.168.1.1 and 8.8.8.8 (as a backup) maybe with DNS it will all magically start working 
Oh edit:
I was reading up on the airport base station, to see how it works because that's the part of your setup I'm not familiar with(if that would be a double NAT, etc) . Anyway I see it supports 100mb and gigabit Ethernet, no mention of 10MB...
today after work I'll try the solution you suggested me, anyway I have a 100MB ethernet board on my mediator... a Realtek RT8139 board...
Anyway yesterday I tried to put another IP address, 192.168.1.33 and it worked!!! I goy response from router!
But when I tried a second ping test is was not working again

ok I did it, I put a static IP address using the macbook one, 192.168.1.104 and the result is the same. But I noticed that if the interface is offline the ping behaves like before (ret=-1), when I put the interface online the shell freezes up...