Do you have another Linux machine, so that you can use dd? If so, then try:
dd if=openwrt-15.05.1-brcm2708-bcm2709-sdcard-vfat-ext4.img of=/dev/sdd
Note that the "sdd" may be something else.
Type ls -lh /dev/sd*
into a terminal, and the last hot plugged device (probably your uSD card) - should have the highest letter, maybe sdc,sdd, sde, etc, but without any "1" at the end - which is a partition. You don't want the "1". You can also use the usb device utility to make sure you have the correct device, since the order could be changed by repeated plugging and unplugging of other devices.
I don't know if the openwrt router has any video capability. You keep saying "blank screen" which seems to indicate you expect the HDMI video to work with openwrt. You should be able to access the openwrt management web server, over the ethernet connection, by typing this into your laptop's browser:
Then, your browser will either connect, or not, but shouldn't usually be a "blank screen". There should be a login user/password
dialog, and after entering correct credentials you should be able to modify the router's configuration, using your browser.
If your laptop is running Linux, and openwrt doesn't default to DHCP, you may have to configure the laptop's ethernet NIC address, with something like:
ifconfig ue1 192.168.1.101 netmask 255.255.255.0
where ue1 is replaced with whatever NIC device you have.