USB Mass storage mode allows you to interface the device memory over its USB interface
as if it were a pen drive.
However, usually the actual memory module is detachable from the device and can be plugged in to the PC USB port with the correct USB adapter. In fact, most machines these days come with a multi-port 'card reader' to let you easily plug in this module.
The Canon IXUS 990 IS seems to use SD/SDHC cards.
You should be able to get a suitable USB adapter for 'mounting' the memory like a USB Pen Drive and transferring data at USB 2.0 speeds.
Many times (typically with older models) the USB port is quite slow (USB 1.1 maybe).
Transferring directly from the memory card would work much faster.
Your model probably has a USB 2.0 interface.
Here is a reference to the User Manual for your camera (you should have a paper copy).
p16 shows how to insert and remove the memory card
pp38-39 show how the memory card can be plugged into your PC with a card-reader
(like I describe above).
p42 shows the memory card and battery cover as item 12.
p158 describes the memory cards supported:
SD, SDHC, MMC, MMCplus, HC MMCplus
It also describes the interface to be
Hi Speed which implies USB 2.0
sadly no note about PTP or Mass Transfer mode selection -- leaves me in doubt