PC not detecting MXL 990?
Me2
Select the USB device for input (i.e., recording) using the Sound applet in the Control Panel. Some versions of Windows have additional shortcut methods. Use a known good USB data cable, and confirm that the power LED is lit. Turn off phantom power. Plug in a keyboard, guitar, or other instrument that uses a 1/4" connector, Set the phones and instrument volumes about midway, then increase the UMC22 gain. If the power LED doesn't light, or you cannot get audio either via the headphone jack or Windows, the UMC22 may be defective. Assuming that the above test was successful, use a tested XLR cable and a dynamic microphone. If this works, unplug the microphone and plug in the MXL 990, turn down the gain, then engage phantom power. If there's no output, the MXL 990 probably has a fault.
Robert J
The PC does not "know" about anything connected to or beyond the interface. If the interface itself is working, you should see a "signal" light when you turn the gain up and have an audio source feeding in to it - the mic or a guitar / keyboard etc. If you connect headphones to it and press the direct monitor switch you should also be able to hear the input directly. If it's not working properly my first thought would be a power problem - a faulty or too long / too thin USB cable, or the USB port on the computer cannot supply enough current, especially with a laptop or if you are using a front port on a desktop case... Is the power light coming on? - and does the brightness of that change at all when you switch phantom power on & off? If you are using a front USB port on a desktop machine, try connecting to a rear panel port instead. Otherwise, do you have any other audio source that you can try, to see where the problem is - and do you have access to another computer, to rule out a USB power problem? Edit - ps. have you tried just rebooting the computer with the interface disconnected, then re-connecting it after the computer has fully started up and there is no disc activity. It could be as daft as the change of drivers has confused things; re-connecting the device after a restart may fix that.
Beans
XLR Cable*