I looked at your diagram.
The link from L3SW to the 2600 should be configured as a L2 trunk.
The link from the L3SW to the L2SW should be a L2 trunk on both sides.
The VLAN 50 should exist in the L3SW and L2SW.
The Fa0/1 physical interface should get a no shutdown on the 2600.
The L2SW's virtual interface, which I don't know how to configure on a DLINK, should be enabled, and the L2SW should have a default gateway that points to the router.
As a test, you may want to create an SVI (switched virtual interface, that is what they are called on a Cisco switch) on the L3SW and see if the L3SW can ping the router, and work back from there.