In our case, we had to attempt to run SizeUp first, have it fail because control wasn’t enabled, and then we found that it appeared in the new Accessibility list in System Preferences. Unlike previous versions of OS X, which used a universal checkbox in an “all or nothing” approach, the new functionality in Mavericks allows users to individually choose which apps can gain control of the system to perform their various scripted functions.
You’ll find it in System Preferences > Security & Privacy > Privacy > Accessibility. Thankfully, after a bit of searching, we found that Apple had simply moved the option… to the Security Preference Pane.
