Demo should be in:
C:\Program Files\Steam\steamapps\common\counter strike source
(Or the equivalent) as it is repectful to your directories and partitions.

Open CS:S, type in console:
playdemo auto

It will auto parse the file name in a menu below the text box. Click the file name and press enter.

You may then type demoui in console to open the panel, or press (I believe) F5 by default.