This is not a mach3 forum, but a forum for the UCCNC software, so it would be better if you email our support at
info@cncdrive.com or
support@cncdrive.com or the reseller shop in case you purchased the device not directly.
But I'm happy to give you some ideas.
Mach3 asks to use the UC100 as long as the UC100 plugin .dll file is in the Mach3/Plugins folder and if the motion controller selection window is not disabled, in other words Mach3 will ask you this question always if the plugin is installed, no matter if the UC100 is not even connected to the computer.
If the UC100 does not show up in the Windows device manager then there is no physical connection between the computer and the USB chip of the UC100.
This means that the computer can't communicate with the device, it can't reach the device.
There should be a "UC100 motion controller" entry in the USB devices section if the device has a connection to the USB port and if Windows recognises the device (USB drivers installed),
or there should be an entry of an unknown device or FTDI device, the exact text depends on the Windows version.
If this is the case then you have to install the USB drivers for the device, because it was not installed or was disabled by Windows for some reasons (For example a bad/errorous Windows update.).
But in any cases, when you plugging the device in and out of the USB port if there is physical connection then you should see the Device manager to update, reload it's content at the time when you do the plugging in/out and then you can compare the list to see which one in the list is the UC100 in case if it comes up as a noname uninstalled device. So you can know that the connection is physically OK.
If the device does not show up at all then the connection is not working. Which can still have several reasons. A broken, or non conducting dusty/loosen USB port, broken USB cable, an electrically damaged UC100.