The XEM6001 is a bus-powered device in its default configuration (R26 resistor inserted). That is, it takes power from the 5-v USB power and generates the voltages it needs from there. To do so, the XEM6001 has small linear regulators for 3.3v, and 1.2v. External power may be applied to any of the 3.3-v pins on JP1, JP2, or JP3 as long as R26 is removed. In this case, the 5-v USB power is not required and the device consumes very little bus current.
Computers and USB hubs often have USB ports that do not provide bus power. These are called unpowered ports. In order to be operated as a bus-powered device, the XEM6001 must be connected to a USB port that provides bus power. You should check with the hub or computer manufacturer to verify that the port provides bus power.
The XEM6001 may also be self-powered by removing resistor R26. This 0-ohm resistor connects the 3.3-v regulator to the 3.3-v supply plane on the board. By removing this resistor, the 3.3-v supply as well as the derived 1.2-v supply are disconnected from bus power. 3.3 volts must be supplied externally — to at least one of the 3.3-V pins on JP1, JP2, or JP3.