Fairly sure that all of the following information is present in available official documentation. However, for the sake of those here having trouble, I'll try to summarize:
- On the computer you are going to use for Netinstall, disable ALL network interfaces (including Wi-Fi) EXCEPT for the ethernet port that you plan to use
- On the remaining interface/port, disable DHCP client and manually set IP address to whatever (pick something, anything; it really doesn't matter)
- Start Netinstall.exe as Administrator (right-click > "Run As Administrator...")
- Click "Net booting" button, make sure "Boot Server enabled" is checked, type in a "Client IP address" that is within the same subnet as what you set the computer'e ethernet interface to, and click OK
- Without applying power to the RB first, connect RB750 ethernet port 1 to computer's ethernet port with any ethernet cable (RB750 and/or most modern computer ethernet interfaces should auto-MDIX even if you aren't using a cross-over patch cable...but if you want some additional "insurance", you could put a dumb switch in between)
- Using a pen/unwound paperclip/nail/tack/other thin object, press and hold the "RESET" button on the back of the RB, and while continuing to hold the button down, apply power to the RB
- Continue to hold the "RESET" button while observing the ACT or USR LED; upon powerup, it will begin with solid illumination, and then as you hold the button, it will start to blink...keep holding the "RESET" button until the blinking STOPS (could take 20-30 seconds!!!) and the LED goes dark for a few seconds, then release the "RESET" button
- After several seconds, you should see the RB's MAC address show up in the Netinstall window's "Routers/Drives" list: click on it to highlight it
- Under "Packages", "Browse" to the location of your "routeros-mipsbe-6.xx.npk" file, select it in the list below that, and click the "Install" button
- Once it is finished, "Install" button will change to "Reboot"...click that, or simply powercycle the 750, and close the Netinstall application
If that doesn't work, then try all of the steps again, but uncheck the "Keep old configuration" checkbox...this will not only format and reinstall RouterOS, but will throw out the old config and reset the router to factory defaults. You will have to reconfigure from scratch, but at least the router will be back from the dead! And if you never see the router show up in step 8, then try Netinstall from a completely different computer.
In the end, it really isn't that hard...and it turns out that it is extremely difficult to permanently brick a RouterBOARD, so kudos to MikroTik for that!
EDIT: Oh, also: make sure you aren't running Windows Internet Connection Sharing (ICS) at all on the machine you are running Netinstall from, and if you are, turn it the @%@#$ off. Netinstall needs to run a BOOTP server to properly chain the install application to the RouterBOARD. ICS runs a separate DHCP server, which could conflict.