I bought Photoshop Elements 11 a few years ago....

Sarah,

I checked your account as I did not see Photoshop Elements registered to your Adobe ID. If you want to send me a private message with the serial # I can check to see if its a multi platform serial #

Thanks

Scott

Sarah,
if your Photoshop Elements 11 is not a multi platform. You will need to purchase Mac version of the software. I would suggest the updated Photoshop 13

Thanks

Scott

you'll need a mac serial number.  if you have that, you're good to go with the installation file:

Downloadable installation files available:

• Suites and Programs:  CC 2014 | CC | CS6 | CS5.5 | CS5 | CS4, CS4 Web Standard | CS3
• Acrobat:  XI, X | 9,8 | 9 standard
• Premiere Elements:  13 | 12 | 11, 10 | 9, 8, 7 win | 8 mac | 7 mac
• Photoshop Elements:  13 |12 | 11, 10 | 9,8,7 win | 8 mac | 7 mac
• Lightroom:  5.7.1| 5 | 4 | 3
• Captivate:  8 | 7 | 6 | 5.5, 5
• Contribute:  CS5 | CS4, CS3

Download and installation help for Adobe links

Download and installation help for Prodesigntools links are listed on most linked pages.  They are critical; especially steps 1, 2 and 3.  If you click a link that does not have those steps listed, open a second window using the Lightroom 3 link to see those 'Important Instructions'.