Button 1 is the button to get past that warning screen.

If that does not work, it seems like your IOS36 is not trucha patched. Use trucha bug restorer and do the following:

note: TBR will reset to the HBC menu after each of these steps, try not to power off your Wii or exit from the HBC during this until you have preloader installed (just for safety really). You will be asked several times where to load the files from, choose NUS (Network update) if you have your Wii connected to your router or alternatively you will need some WAD files, read the TBR wiki if you want more information on those.

1. Select IOS36, downgrade IOS15.
   
2. Select IOS15, go to the IOS36 menu, install patched IOS36 (trucha + identify) to IOS36.
   
3. Select IOS36, restore IOS15.
   

Once you have done this you should be fine to go ahead and install preloader. It seems like your friend did not really do much softmodding for you or your wii was updated via nintendos servers and it has deleted all your hacks. Preloader will stop this happening in future.

DO NOT UPDATE YOUR WII VIA THE ONLINE UPDATE OR FROM A GAME DISK