1. For CYPD3120, the application based on this part number is C to DP dongle with USB billboard. If you want to develop your firmware based on this application, you should use the example code in the link of SDK3.0.2.
And the PSoC Creator is PSoC Creator 3.3 DP1. Please download the package of the SDK includes PSoC Creator 3.3.
If you want to use the CYPD3125 notebook example code, please kindly make sure it is DRP design with DisplayPort Source firmware。
For your question.
1) Where and how can I add code?
>> The place you can add is file app.c. This is application layer of the project.
2) For example at top there is TopDesign.cysch file. What is this file for?
>> The Schematic Editor displays the top-level schematic file (TopDesign.cysch) as a document window, and the Component Catalog opens to display a list of components to use in your design. Refer User Guide of PSoC Creator:
3) How can I get the memory size after compilation? Because I want to know how much emty space I have.
>> You can re-build the firmware with PSoC Creator directly, you will get the memory size.
For all of your question, you need read get start of PSoC Creator at first. If you have been installed PSoC Creator, you can get the documentation from HELP>>Documentation.