Scroll through the options until you see the LetterBomb folder.
Click on the Wii message board icon at the bottom-right corner of the main menu. Note that the SD card slot is located next to the disk drive or under the disk drive, depending on how you have your Wii set up. The main purpose of this feature is for rapid development of homebrew(well, as rapid as you can develop for a console like the Wii U), but you can basically use this feature to launch homebrew elf's of any size, including mocha, sent from your pc, which should allow you to redirect SD card access to USB in theory. Insert your SD card into the Wii console. You won't see any homebrew listed for obvious reasons, but the homebrew launcher will start a tcp listener in the background, which receives and executes homebrew elf's over the network.
The homebrew channel (installed separately from haxchi!) will start without an SD card mounted because it is installed either on NAND or a USB device. If launching haxchi directly redirects you the homebrew launcher (not the channel!), you use a custom config and need to find out which hotkey you need to press while launching haxchi to get back to the system menu with sigpatches. The standard haxchi config returns the user to the system menu with signature patches installed, allowing you to launch the homebrew channel if installed separately before. I'm sorry if I didn't explain myself properly.