Most Spectrum software assumes you have a keyboard attached (since the original Spectrum is an all-in-one home computer). FUSE for the Wii supports USB keyboards. You will probably need one.

When you run FUSE, you get the Spectrum 48K boot screen. You can now attach a USB keyboard and write BASIC programs!

You can also push the HOME button on your Wiimote. This brings up a menu that you can use to load Spectrum games from SD card. The A button selects a menu entry, the B button brings you up one menu. You should also tell FUSE to use your Wiimote as a Sinclair Joystick: Options->Joysticks->Joystick 1->Sinclair Joystick

Now, when you start a game, tell the game to use the Sinclair interface. Hold the Wiimote sideways so the 1&2 buttons are on the right. The directional pad is your joystick, 2 is Fire button 1, 1 is Fire button 2.

To quit FUSE, bring up the menu and select File->Exit, press A on the question whether you really want to quit.