How do I activate Google Custom Search? I've just installed the Google Custom Search Module.

I've activated the module and inserted the API. The search is is till not working.

Something I'm missing?

Note: I've realized I need the paid version. is there a free Google Custom Search, free option?
As you can read in README.txt file:

*** About Google Site Search module
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This requires "Site Search Edition" (paid version of Google's Custom Search).


You need to have a paid Google Site Search engine, so you can receive results in custom format (e.g. XML).

If you've free version, check the alternatives modules such as: Google Search Appliance or Google Custom Search Engine. Or use alternative services such as Yahoo YQL (e.g. YQL Views).