With the latest releases of the Mac OSX operating system, there is a new security feature called "Gatekeeper". Gatekeeper prevents the user opening applications not signed by Apple. There are two workarounds for this:
1) Hold down the "CTRL" key and select "open" when opening the installer.
If you have done this correctly you will now see an option to open the installer.
2) You can also deactivate the Gatekeeper by going to Apple > System Preferences > Security & Privacy. You can then click the padlock at the bottom of the window to make changes.
Allow the applications to be downloaded from: 'Anywhere'
At this point current releases of Serato software are not signed, so you will have to bypass the Gatekeeper function. For more info check out what Apple says about Gatekeeper here.