If you get an error similar to the following after installing Remarkety (or any other component as a matter of fact):
Fatal error: Class 'Remarkety_Mgconnector_Model_Resource_Setup' not found in public_html/includes/src/Mage_Core_Model_Resource_Setup.php on line 234
The problem is that, if so configured, Magento keeps a "compiled" version of all the PHP files, for performance reasons (read more here). The problem is that the new extension files haven't been copied to the "compiled" directory, and Magento can't find them.
To make Magento use the regular files instead of the compiled version, you have 2 options:
1. If you have SSH access to your server, run the following command:
php shell/compiler.php disable
2. If you don't have SSH access, edit the file: includes/config.php (either through your cPanel or by FTP), and make sure the first define is commented and the second is not, like so:
Either way, at the end also remove the contents of the var/cache directory.
If you really want to use Magento's "compilation", you can turn it on again by:
1. Login to the backend
2. Navigate to System > Tools > Compilation
3. Click "Run compilation process"