You don't change channel on your computer you change it on the Homehub and your computer will automatically change onto it.
See link how to do that.
Yes, I understand now. It must be something to do with the browser that you are using on the iMac. I don't have an iMac so can't try it to see what the problem is.
Which browser are you using on the Mac and is it running any flavour of ad-block?
I'm fairly sure I can remember that the Home Hub 1 GUI page had compatibility issues with Safari (it's a long time since I've come across anyone using one! )
Try using Google Chrome instead and it should work fine.