Hmm. I read on a review that someone could only get 1440x900 while using the VGA cable, so I thought I'd lead that way, but if you've tried both??? I'm at a bit of a loss.
You could try installing a driver that comes on the Support Disk for the Monitor:
1) Put the support disk in your drive.
2) Open your Device Manager and expand the Monitors section. Double click on the available monitor and choose the Driver tab, then click Update Driver. Choose Not at this time, Next, Install from a list or specific location, Next, Don't search. I will choose the driver..., Have Disk...
and point it to your Support Disk. You may have to try a few different directories until you find an .ini or .inf file.
Sort of my last ditch effort to help. Good luck to you.