Please read section 7.7, and 7.7.1, in README.TXT. It doesn't get installed by an update, because it is still experimental software, and update does not add features (with very few exceptions). It does get installed with a new install. I really don't know why they did it that way.
I expect that USB 3 will be formally released via the subscription channel, when it is actually completed. At the moment, it is included as a convenience for those who don't have any choice but to use USB 3 for install. It may not work, depending on what adapter you actually have, I have a Fresco Logic adapter, that will work, if the conditions are perfect. The main problem with it is that there is no additional power available, and many devices draw too much power, causing it to turn off. Even plugging in my powered hub can cause it to power off. There just isn't enough power from a PCI-e connector. This is not unique to OS/2, it also happens in windows. If I leave the powered hub connected, and powered, while I boot, it will work (when the connector makes proper contact. The connector doesn't work well).
I will mention that USB, in general (not just USB 3, and not just OS/2), is a terrible technology. Nobody seems to follow the rules, and the connectors (especially A type connectors) are really flaky. If there is anything, at all, wrong with them, they are going to cause problems. USB 3 is more likely to have problems, because there are more contacts that have to connect properly, or various inconsistencies will happen. The usual ones are:
The USB 3 device will connect at USB 2 speeds, and not at USB 3 speeds. Many causes.
The device will connect okay, then some time later it will disconnect itself, for no apparent reason (usually somebody stood up, or moved a chair, which jiggled the connector enough to break the connection). Heavy devices, or cables, may drag the contacts apart, causing disconnection.
USB extension cables usually have worse connection problems than plugging a device directly into a fixed connector on the machine (two connectors to fail, plus they may move, which can break the connection).
USB HUBS can also have the connection problems. Since USB 3 HUBS actually have two devices (one for USB 3, and one for USB2), they may work with one, or the other, but not both. To work with both (they should), both sets of contacts need to be perfect.
The physical connectors on computer, cables, and device, need to be near perfect. Unfortunately, there is no easy way way to actually tell if a USB 3 device connects at USB 2 speed, and/or USB 3 speed. Speed depends on many things, and most of my USB 3 devices are not noticeably faster while connected to USB 3. If you pay for premium devices, they should perform better, as long as the contacts work. If you pay for cheap USB 3 devices, they may work better when attached to a USB 2 controller. On the other hand, some USB 2 devices will actually perform better, when attached to a USB 3 controller (actually two controllers, one for USB 3, and one for USB 2). You may need to try different combinations.
Have fun...