I found the solution online here: dltr.org and I’m posting it on my blog to make it easier for me to find later on!
Download a new copy of Magento from the Magento Commerce site and unzip the file.
Open Magento > lib > Varien and copy the Simplexml directory, then navigate through your sites on your localhost to the same location and replace the directory by pasting and overwriting.
Restart MAMP (or XAMP, LAMP, WAMP etc) and reload your sites.
Thanks to Damodar Bashyal for this!