MRX300 does not have a USB port. However all firmware on the MRX300 is update over the serial port on the back of the receiver. This is the same serial port used by ARC to do your room correction measurments.
If you don't have a serial port on your computer, you will need to get a USB Serial adapter. Details about the recommend (and strongly advisable to only use the recommended) USB to Serial adapter are listed in the FAQ post (post #2 again).
The MRX's come with a very nice serial cabled but do not come with the USB adapter (or at least they didn't used to). You might confirm with your dealer that they still do not have an included USB Serial adapter and that your dealer is not already including one (I know a few dealers include one, though I suspect they also include the cost in the price they quote). If your MRX is not coming with one and you need one, I would order it today so it arrives with your MRX!
EDIT: Strange, I total missed Shrike645's response to you when I posted this... yet he responded like 5 hours ago!