What you have to do is creating Automation for that parameter. In GarageBand that automation data can only be created "offline", meaning you have to create it manually by drawing the automation curve.
Here are the steps:
- Click on the Show/Hide Automation Button above the Track List to the Automation components visible
- Enable Track Automation on the Track Header where you want to create automation
- Click on the Automation Parameter Button to select which parameter you want to automate
- Hold down the command key and click (or drag) an automation curve for that parameter
- In my example, I selected the "Low Cut Frequency", This requires that that frequency band is enabled in the Equalizer Plugin
- When you playback your track, you will see the changes performed "live". You will see how the frequency band moves, depending on the automation data

Here is a look at the Automation Parameter Menu:
The menu is grouped in four sections:
- Main Parameters
- Smart Controls: These are the parameters in the Smart Controls Window of the Track that can be automated
- Plug-ins: This section lists all the Plugins that are currently loaded on that Track, The "Channel EQ" is one of them. In its submenu, you can find all its Parameters
- Used Parameters: The last section lists all the Parameters that are currently automated on that Track

As you can see, there are a lot of details when it comes to automation. I cover that and all the rest of the GarageBand in great details in the only graphically enhanced manual "GarageBand X - How it Works"
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.