assuming we talk about CC, you will find the download page there:
http://www.adobe.com/downloads.html?promoid=JZEFS >>> https://creative.adobe.com/products/photoshop?promoid=KHTUU
Other versions like Photoshop Elements products | 9, 8, 7 for example you can find here
What concerns the installation problems you could have a look there:
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My copy is Photoshop Extended CS6 and it's on a disk. My new iMac doesn't take disks.
what I would like to say too with my answer: You "must" be able to load down a Trial Version of CS6 to your mac and by using your serial number.
Download Photoshop CS6 Extended from this link
Install then enter your serial number to license it on your Mac.
Note: your existing serial number must be for a Mac. A Windows serial number will not work on a Mac.
If you previously purchased a Windows serial number then a platform swap will be required.