Make sure you've installed the samsung usb modem driver. Make sure your phone is completely off. Make sure, in activesync, you have unchecked "allow usb." Open UMDL.
Step 1 - Click on PDA button (in PDA section) and then select the bin file you are trying to flash.
Step 2 - Make sure the following are checked: single download mode, launch after download, start downloading immediately.
Step 3 - Connect your phone to your PC. Click Detect (on UMDL). Turn on your phone. At this point, it should start downloading. UMDL will show your progress. Don't touch anything until download is finished. Your phone should boot up automatically. At this point you can exit out of UMDL and disconnect your phone.