When installing the ChannelUnity module for Magento, if the Compiler is enabled in Magento, this can cause issues so it is necessary to temporarily disable it, just while you install the module.
First login to your Magento Admin panel and then follow these steps.
Go to System -> Tools -> Compiler.
Once there, you should see a "Disable" button.
Click this and you should be able to see that the compiler is now disabled.