If it is an MXL 990 Condenser that comes in the grey case with a shockmount and a black hard mount I can solve your problem. (If it is USB I know nothing about that unit).
This MXL 990 microphone MUST have 48v phantom power in order to operate. If you aren't familiar with the term phantom power read the first link:
Phantom power - http://en.wikipedia.org/wiki/Phantom_power
MXL User Guide - http://www.mxlmics.com/support/guides.html
If yours is one of the older MXL 990's it should have an XLR connector on the bottom -
XLR Defined - http://en.wikipedia.org/wiki/XLR_connector
Your mixer (I am guessing its a Peavey, Tascam, Mackie) or something entry level like that may or may not have phantom power. I am guessing not. But if it does, it will have a switch that says 48v phantom power and most likely a green or red LED next to it to show you when its on. This mic will NOT work without phantom power.
Best suggestion: Verify that your mic is the MXL 990 Condenser, if it is it needs 48v phantom power. Check your mixer and see if it has 48v phantom power. The only computer soundcard that I know of that is used by a lot of folks is the M-Audio Delta 1010LT. It has XLR inputs but NO phantom power on it.
There's your answer.