As seen in the 3DS Mii topic, here is my tutorial for how to transfer your Mii's from your Wii to your 3DS.

• Open up Mii Maker on your 3DS system.
  
• Choose the Send/Receive Option
  
• Open up Mii Channel on your Wii Channel

There you will see instructions on how to set up the Wii for transfer.

• On the plaza press the 'Connect to DS' button (If you don't have that option, press A, then B, then 1 and hold 2.)
  
• Your Wii will automatically show up your 3DS.    
  
• Now follow the on-screen instructions.

    
Some people might already have the option. I guess this comes with a system update higher than 3.4.

You can send your Mii's from your Wii to 3DS, but you can't send them from your 3DS to your Wii due a lack of compatibility since the 3DS offers more editing features.