First, using Airport Utility from ANY machine on your network, make sure the 5 GHz network is ENABLED on your Extreme. Click on the Airport tab, then select Wireless, then Wireless Options. Name your 5 GHz network (must be a different name than your main network). Click ok, and then update your Extreme.
Once your Extreme is updated, go to EACH individual machine that is n-capable and join the new 5 GHz network that you just created (assuming all your machines are Macs, go to System Preferences >> Network, or just click on the wifi icon in the top right, and join this new network).
By doing this, all n-capable machines will run at 5 GHz while everything else will be on the 2.4 GHz network. You are essentially running two separate networks.