1. Please connect it via RJ45 ethernet cable, go to your Router, take the power plug out, and put & hold a pin/paperclip into the reset button. Put power on (keeping the paper clip in) and the front light should start flashing orange in a couple of seconds.
2. At this point the router is in failsafe, emergency mode
3. On your PC (macs are similar though), set your IP Address to 192.168.0.10, subnet 255.255.255.0
From a browser (i used chrome and it worked fine, but I hear IE might be better), navigate to 192.168.0.1
This takes you to the "emergency room - flash loader)" website. At this point you can provide the factory.bin and voila flash away.
4. Leave it 2 mins or so.. When the flashing is done the router will reboot and the lights will flash green. At this point reconfigure your PC to use DHCP and then navigate to 192.168.1.1 to access openwrt or DD WRT.
Dir-615 D4 OPENWRT or DD-WRT Firmware Download: