1. Download your system menu version with NUSD (check pack->wad). Put the wad in the Wad folder on the SD card
2. Boot into Priiloader
3. Launch the Homebrew Channel
4. Run the wad manager, let it use IOS36 (or IOS236) if it asks and install the system menu Wad
Or just wait for Mauifrog.
IMPORTANT: BEFORE installing any system menu, you MUST make sure to install a NON-STUBBED version of the IOS that system menu version requires!!! If you do not, you WILL brick your Wii!!!