USB uses differential between the D+(Green) and D-(White) lines. You will need to use two probes:
Connect channel A to D+ with probe ground lead to Ground(Black) and
connect channel B to D- with probe ground lead to Black.
Both probes should be switched to the X10 mode and X10 selected under the 'channel options' in the software for each channel.
In order to pick up communication the host computer and device you are monitoring have to be actively communicating with each other for example if you want to monitor a USB pen drive, you will need to do it whilst it is transferring a file. You will not pick up anything from a wire connected to a computer with no device attached/communicating.
While the device and computer are communicating, you should press the Auto-setup button to set up the ranges of the device. The signal should be around 3.3 V and channel B will be an inverse of channel A. If you do not see 3.3 V on the data lines, you may need to probe at a different point.
Set the Trigger mode to 'Auto' and then set up the serial decoder with DataPlus: A and DataMinus: B. The thresholds will be automatically set up if the voltage levels of the D+ and D- are being displayed correctly.
You can change to a shorter collection time such as 100us/div to see the packets more clearly.
Hope this solves the problem.
Technical Support Engineer