Hi adriangl,
have you read John's instructions in the other forum thread: http://scratch.mit.edu/forums/viewtopic.php?id=2745 ?
once you have downloaded the Scratch source code you can start it by dropping the *.image file on the Scratch.exe (or .app) to open it.
But remember, you don't really 'need' to do anything with the source code to get the most out of Scratch, because there really aren't (m)any secrets in that code. Experimenting with the source code of Scratch is great if you want to understand how Scratch is made and how exactly it works. You can actually learn a lot by just browsing the code without changing anything.
P.S. I moved this thread from Troubleshooting to the Advanced Topics forum section
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