Ok I just purchased Simcity 4, now how do I play the game, where is it on my pc

The only anwer I can give you is on the hard drive - hopefully someone else has a better one.

more details from you might help.

You took an action to buy the game. What action did you take to get the game on your PC?

Your two main options are:
1) Download and install it your self via the account page ("Account" tab on the web page navigation bar)
2) Install Galaxy, log in, let it install the game for you.

If you haven't done either of those things (or have no idea what either means) then you don't have the game on your PC yet.