Plug your Alesis i026 into an available FireWire port on your Mac. If you do not have FireWire ports, you will need to use a Firewire-to-USB adapter and plug the i026 into a USB port.
Open GarageBand. It should appear in your "Applications" folder and feature an image of a guitar as its icon.
Click "GarageBand" in the menu bar at the top of your screen and select "Preferences" from the drop-down menu.
Click the "Audio/MIDI" tab at the top of the "Preferences" window.
Click the menu that appears next to "Audio Input" and select "Alesis i026" as your input device.
Change your "Audio Output" to "Alesis i026" if you want to use your Alesis i026 for playback. You will need headphones or speakers for this option. Leave your output device as "Built-in Output" if you want to use your computer for playback.
Click the red bubble at the upper-left corner of your "Preferences" window to close your preferences.
Plug any instruments, microphones or MIDI devices into your Alesis i026.
Create a new GarageBand audio track by clicking the "+" button that appears in your browser directly beneath the tracks list. When the pop-up window appears asking you what type of track you would like to create, choose "Real Instrument Track." This will enable you to record onto GarageBand through your Alesis i026.
Press the "Record" button that appears beneath your tracks list. It looks like a red circle. Make sure that your new instrument track is highlighted when you begin the recording process.
Begin recording by playing the instruments or singing through microphones plugged into your Alesis i026.
Press the "Record" button again when you finish.