I have tried Opera, IE 8.0, Firefox 11. Same thing.
I don't see any settings in there that could possibly prohibit a user to set IP Range in the Traffic Control section.
As for comparison, I have the older DIR-600 with specialized firmware version from DLink for my country (Indonesia) that has the same Traffic Control option. And I can set the IP range without a problem with exactly the same settings as DIR-600L.
The reason I need this section is because I'm using 4 Mbps cable connection, which I would like to divide into 2 computers, each with a fair share of 2 Mbps max bandwidth.