PC has internet access, MAC does not, both network access

I've got an Edimax wireless router BR6104WB (firmware version 2.65).
There's an ADSL-modem connected to this router (Speedtouch).

Two computers are connected to this network, a PC and a MAC.

Both the PC and the MAC have access to the network. However, only the
PC has internet access. The MAC does not.

Using the MAC, I am able to ping to the modem (10.0.0.138). I can also
reach the PC with the MAC.

Connecting the MAC to the router with a cable has no effect.

If I connect the MAC to the ADSL-modem directly (so no router in
between), the MAC does have internet access.

(Seems to me that the problem is in the router. But where?)

What can be the cause of this problem and how can it be solved?

Thanks in advance.

Greetz,
Ronald