Mac OS 10.8 Mountain Lion introduces Gatekeeper. This is a security feature that requires each non-Apple software installer to be Digitally Signed - if an installer is not Digitally Signed Gatekeeper will not allow the installation to proceed. SSL are in the process of rebuilding all our installers with a Digital Signature.
In the meantime, you can turn off Gatekeeper in the following way which will allow you to install our current Mac software on Yosemite, Mavericks + Mountain Lion:
Navigate to System Preferences>Security and Privacy
Under the 'General' tab, change 'All Applications downloaded from' from: 'Mac app store and identified developers' > 'Anywhere'
Now click 'Allow from Anywhere' in the following box.
Alternatively you can leave leave Gatekeeper active but bypass it on each individual installer by control-clicking its icon and selecting Open.
Please note: After upgrading to Mavericks Gatekeeper is reset to default settings. After you perform the upgrade, please follow the steps above to turn it off again.