Assuming you are using 'full screen mode', when you hit the ALT key the sim pauses and you get the menu across the top of the screen. Towards the right you will see View - click on it and you will get the list of view options.

Alternatively, if you are in Cockpit view, hitting the S key cycles the views - the first strike puts you into the VC (the next into Tower view, then Spot view then back to Cockpit)

Hope this helps