You may find this tutorial to be of great help:
I'll try to walk you through the process:
1. Launch Garageband and select a piano template.
2. In Garageband > Preferences [Audio/MIDI] you can set the default Input and Output for GB.
3. Under the [Advanced] tab set the Audio resolution to Best.
4. Create a new 'real instrument' track by clicking on the + button in the bottom left of GB 09.
5. In the Inspector area (right side) you can select the Alesis as the Input source and turn 'Monitor: On'.
6. Playback from your tape deck to set the recording level appropriately, so the main level meter does not go into the red area.
7. When you're ready, start recording in GB (you may wish to turn off the metronome first in the Control menu) and then press Play on your tape deck.
That should now begin recording the stereo signal from your Alesis into Garageband.
Once you're finished recording press stop, begin editing the audio and when you're ready to export use the 'Share' menu where you can choose to Share direct to iTunes or many other destinations.
Hope that helps, & let me know how you get on or if you need any further help.