1) Launch Garageband, create a audio track
2) Go to "View" -> "Open Smart Controls"
3) Click on the "i" icon on the smart control top bar, then drag down the scroll bar to find Audio FX slots. Remember to click on "Plug-ins" to unfold the Audio FX column.
4) Click on the Audio FX (It might be separated to many effects already) slot and find BIAS FX from 'Audio Units -> PositiveGrid -> BIAS FX'