The original µSD card mentioned is the one internal to the Kobo Glo. You will have to remove the back cover to access the µSD card. Once you have the card extracted, you can mount it on your computer (if you are using Windows, note that the card has 2 Linux ext4 partitions and Windows will want to format the card. Do not format the card. After that is done with, you should see the FAT32 partition mounted. You can copy the unzipped firmware into the .kobo directory, reinstall it in your Glo and when you power your Glo on, the firmware should install.
Given the USB computer detection just requires the V+ and gnd connections on the USB cable, it is possible that your Glo's USB port is damaged. You can try cleaning the port to see if that helps.
I've attached an image of a Glo with the back off and the battery removed.
See this thread for advice on removing the internal µSD card:
Memory Upgrade Mod Guide (With Pictures)