Hold the reset button when booting the Wii to load Pre/Priiloader. Currently I don't believe there is another method to booting straight to/from USB Loader.
You'll need to have the forwarder dol installed to the root of your SD card, which can be obtained from the Readmii for USB loader Here.
Specifically, taken from the Readmii,
[1c] [Optional]Autobooting and returning to USB Loader GX
It is possible to boot your wii directly into the USB loader GX. The only thing you need for this is a wii app called preloader. First download the newest rev of the loader or even better, the forwarder (dol format, see downloads at the top of the readmii). Place the dol you get in the root of your SD card (no need to rename it to boot.dol). In the preloader menu, choose install file and choose the dol to install. When its done, go to the main menu of preloader (B) and then choose 'settings'. You have to change 2 options here: Autoboot=file and Return to=preloader. Save the settings, then launch the Wii menu in the main menu. It will boot into the USB loader GX!