Download and install CS5.5

CS4, CS5 and CS6 are on life support.  They are not supported (use at your own risk) but can still be activated providing you have the following:
1. original media (disks or downloads) you received at purchase,
2. a legitimate serial number for that particular media, version and platform (Win/Mac),
3. an older operating system that can support legacy Creative Suite.
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NOTE 1: Creative Suite installers won't work on macOS Sierra, High Sierra or Mojave without terminal workarounds.
https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

NOTE 2: Catalina is not an option as it cannot support older 32-bit apps.

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Beware of fake Adobe reps who may contact you privately offering to sell you software or take control of your computer. It's a scam, run away!

For CS6 back to CS4 you may download ONLY if you bought direct from Adobe via download
-Log-in to your account and click this link https://account.adobe.com/products
--click Order history on the left and then the link to your order that will show on the right
--and finally click the link "Downloads and serial numbers" that will appear
--as always, write the serial number in a text file AND copy that text file and the
---download file(s) to an external drive (or two) to be sure you have what you need
-if you did not buy direct from Adobe via download you must find your original install
--if you have an Education or volume license you MUST use your original media
--the link for an Education or volume license download is not available at all
-if you bought a disc and don't have a drive you will need to buy an external USB drive
-if you bought a download from a 3rd party vendor you will need to contact that vendor