I use a Mac OS X machine and the Harmony 900 didn't work when I plugged it into the USB port. It turns out that it uses the USB as a network adapter. To set up the remote and let the software recognize it, you must follow some instructions from a Harmony troubleshooting website which applies to the Harmony 900/1100 series.
Basically you need to set the local side of the USB Ethernet to 192.168.142.1 with a netmask of 255.255.255.0 and you should be able to talk to it with the Harmony Remote configuration software. You can confirm that the remote is connected by pinging 192.168.142.3
Note: Ignore the advice about setting the "Router" address. This will only cut off your Internet connection whenever the Harmony remote is connected!
The drivers for the Harmony remote are only available in 32-bit mode, so you must boot in 32-bit mode by holding down "3" and "2" buttons while the machine boots. See the Logitech article about it. You can also permanently boot into 32-bit mode by doing this in a terminal.
sudo systemsetup -setkernelbootarchitecture i386
I bought a Xantech starter kit from eBay a year ago in an effort to improve the Harmony 880's operation. Unfortunately, it didn't help. However, you can easily connect the Harmony 900's RF to IR conversion block to a Xantech system:
I used the Xantech 789-44 Connecting Block as the part that does the actual IR blasting.
I recommend using a hot glue gun with low temperature glue to stick all the emitters to everyone. The double-sided tape that comes on most emitters just doesn't stick that long.
If you need to remove the emitters later, simply using a pocket knife and apply a little twisting pressure under the edge of the emitter. It should pop off without leaving a scratch or residue.