Open Audio MIDI setup in your Applications/Utilities folder and from the Audio menu, select Open Aggregate Device Editor. In OS X 10.7 (Lion), click the + at the bottom of the Audio Devices window and choose Create Aggregate Device.
Click on the “+” to add an aggregate device. Select your Blue Snowball mic as the input and your regular soundcard as the output. If you use your Snowball away from your regular soundcard, like when you’re on the road, you may also want to create another aggregate device with your output set to the “Built-In Output” and input set to your Snowball. You might want to rename your aggregate devices to something that’s easily recognizable. Click Done and close the Audio MIDI Setup.
In Logic Pro, open the Audio Preferences menu: Logic Pro > Preferences > Audio.
Your aggregate device is now an available device in the Core Audio menu of your Audio Devices. Select it and relaunch. Inputs 1-2 will give you input from your Snowball mic.