how to use variables in cp ?

That is a very short question. Which variables: system or user variables? Using for what: to show the value, to calculate, to.... Do you want to do this with advanced/shared actions or with JS. Here are some links that could be interesting, although some are pretty old:

http://blog.lilybiri.com/curious-about-variables-in-captivate-4-5

http://blog.lilybiri.com/unleash-the-power-of-variables-in-captivate-5

http://blog.lilybiri.com/system-variables-in-captivate-8