The problem is that hardware package is in the Arduino IDE 1.0.x format so it's not compatible with the recent versions of the Arduino IDE. This issue was reported:
But unfortunately it appears that project has been abandoned years ago.
You should be able to use it with Arduino IDE 1.0.6, which you can download here:
It should also be fairly easy to update the hardware package to work with Arduino IDE 1.6.x and newer. You will find a guide to the changes that need to be made here:
The immediate problem is that the 1.5.x hardware package format requires the package to be within an architecture folder. So you're getting this error:
Could not find boards.txt in C:\Users\Admin\Documents\Arduino\hardware\microbridge\bootloaders. Is it pre-1.5?
because it's treating the bootloaders folder as the architecture folder, which is wrong. That's an easy fix, you only need to create a folder C:\Users\Admin\Documents\Arduino\hardware\microbridge\avr and then move everything from C:\Users\Admin\Documents\Arduino\hardware\microbridge to C:\Users\Admin\Documents\Arduino\hardware\microbridge\avr. After doing that and restarting the IDE the board will appear in the Tools > Board menu but you will likely encounter some other errors. The solution for those are explained in the link above.