You can download the CS5 Trial version and use it for free for 30 days without a serial number.
Follow the Very Important Instructions on that page first BEFORE you click the download links.
Install it and it will run in Trial mode. It will cease working after 30 days unless you enter a CS5 serial number.
Each time I tried to download CS5 for Mac file on that site it says I do not have access to it.
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Follow the Very Important Instructions (step 0 to step 6) first.
THEN click download.