gabi68,
You are right: the USB UC300 will not install with the drivers in the zip or in the installed UCCNC. Download the second link (the exe). Disconnect the PC from the internet. Connect the UC300, open device manager, find the UC300, right-click, select remove, check the checkbox to remove the driver files. Disconnect and reconnect the UC300. Now in device manager you'll see it under unknown devices. Start the automatic installer exe, select Mach3, on the next page select only the UC300, click install. If you don't have Mach3 installed then it will try to install it, but click cancel to skip this. Now it will successfully install the driver. In the next step the installer will try to copy the plugin into Mach3. If the Mach3 folder does not exist then it will throw an unhandled exception, press quit. Now the driver is installed and UCCNC will run with it. (I've just tested the whole procedure.)
Balázs,
Certainly it's my fault, but I tested the drivers only with UC100 and not with UC300.