How to make a demo video using a Mac?
There are two parts to this:


Preparing your content: screenshot the pages you want to present. Use Cmd+Shift+4 to get all the screens. Don't worry about getting some incorrectly, just do a few tries is you have to, you can filter them out later.
Build a presentation: create slides, add highlights and transition animations,  time it just right and practice your speech. You can use Apple Keynote for this, or any other presentation software. 
Record your show: do your screencast in "Present" mode and record your screen. Open QuickTime player and use the option File → New Screen Recording to get some (limited) editing options after you record it. On the start screen, set the audio recording options to pick up or ignore your microphone


If you want to fine-tune the result, or to export it to other formats, make an iMovie project out of it. 

Enjoy :-)