If when connected with the DI-604, you don't get the speed from the plan you paid for, I'd keep on Verizon to resolve. The only thing you need to do for it to be setup properly is to set the PPPoE ID, password and MTU. See http://support.dlink.com/emulators/di604_reve/h_wan_dhcp.html for the example page but the MTU is incorrect on it.
If you are running Windows, you can go to http://www2.verizon.net/help/fios_settings/check.asp and have it set the three registry settings.
If this does not resolve the issue, it's up to Verizon to resolve and I agree with Ronnie, replacement of the DI-604 would be a good next step.